No built-in scheme name for syntax is causing the app breaking

[diegoulloao]

Hey, just testing the custom themes feature you've just released! Making my own version of neofusion theme for Posting :)

I just realized of this bug: if I set a theme name for syntax that's not built-in with the app, it crashes.

Traceback

╭───────────────────────────────────────────────────────────── Traceback (most recent call last) ─────────────────────────────────────────────────────────────╮
│ /Users/diego/.local/pipx/venvs/posting/lib/python3.12/site-packages/posting/widgets/text_area.py:218 in on_mount                                            │
│                                                                                                                                                             │
│   215 │   │   self.register_theme(DRACULA_THEME)                                               ╭────────── locals ──────────╮                               │
│   216 │   │   empty = len(self.text) == 0                                                      │ empty = True               │                               │
│   217 │   │   self.set_class(empty, "empty")                                                   │  self = ResponseTextArea() │                               │
│ ❱ 218 │   │   self.on_theme_change(self.app.themes[self.app.theme])                            ╰────────────────────────────╯                               │
│   219 │   │   self.app.theme_change_signal.subscribe(self, self.on_theme_change)                                                                            │
│   220 │                                                                                                                                                     │
│   221 │   def on_theme_change(self, theme: Theme) -> None:                                                                                                  │
│                                                                                                                                                             │
│ /Users/diego/.local/pipx/venvs/posting/lib/python3.12/site-packages/posting/widgets/text_area.py:222 in on_theme_change                                     │
│                                                                                                                                                             │
│   219 │   │   self.app.theme_change_signal.subscribe(self, self.on_theme_change)                                                                            │
│   220 │                                                                                                                                                     │
│   221 │   def on_theme_change(self, theme: Theme) -> None:                                                                                                  │
│ ❱ 222 │   │   self.theme = theme.syntax                                                                                                                     │
│   223 │   │   self.refresh()                                                                                                                                │
│   224 │                                                                                                                                                     │
│   225 │   @on(TextArea.Changed)                                                                                                                             │
│                                                                                                                                                             │
│ ╭─────────────────────────────────────────── locals ───────────────────────────────────────────╮                                                            │
│ │  self = ResponseTextArea()                                                                   │                                                            │
│ │ theme = Theme(                                                                               │                                                            │
│ │         │   name='neofusion',                                                                │                                                            │
│ │         │   primary='#e8e5b5',                                                               │                                                            │
│ │         │   secondary='#fd5e3a',                                                             │                                                            │
│ │         │   background='#06101e',                                                            │                                                            │
│ │         │   surface='#052839',                                                               │                                                            │
│ │         │   panel=None,                                                                      │                                                            │
│ │         │   warning='#e8e5b5',                                                               │                                                            │
│ │         │   error='#fd5e3a',                                                                 │                                                            │
│ │         │   success='#35b5ff',                                                               │                                                            │
│ │         │   accent='#66def9',                                                                │                                                            │
│ │         │   dark=True,                                                                       │                                                            │
│ │         │   syntax='neofusion',                                                              │                                                            │
│ │         │   author='Diego Ulloa',                                                            │                                                            │
│ │         │   description='neofusion theme for Posting blending lava red and ice blue colors', │                                                            │
│ │         │   homepage='https://github.com/diegoulloao/neofusion.posting/'                     │                                                            │
│ │         )                                                                                    │                                                            │
│ ╰──────────────────────────────────────────────────────────────────────────────────────────────╯                                                            │
│                                                                                                                                                             │
│ /Users/diego/.local/pipx/venvs/posting/lib/python3.12/site-packages/textual/widgets/_text_area.py:723 in _watch_theme                                       │
│                                                                                                                                                             │
│    720 │   def _watch_theme(self, theme: str) -> None:                                          ╭────────── locals ──────────╮                              │
│    721 │   │   """We set the styles on this widget when the theme changes, to ensure that       │  self = ResponseTextArea() │                              │
│    722 │   │   if padding is applied, the colors match."""                                      │ theme = 'neofusion'        │                              │
│ ❱  723 │   │   self._set_theme(theme)                                                           ╰────────────────────────────╯                              │
│    724 │                                                                                                                                                    │
│    725 │   def _app_dark_toggled(self) -> None:                                                                                                             │
│    726 │   │   self._set_theme(self._theme.name)                                                                                                            │
│                                                                                                                                                             │
│ /Users/diego/.local/pipx/venvs/posting/lib/python3.12/site-packages/textual/widgets/_text_area.py:737 in _set_theme                                         │
│                                                                                                                                                             │
│    734 │   │   except KeyError:                                                                 ╭───────────── locals ──────────────╮                       │
│    735 │   │   │   theme_object = TextAreaTheme.get_builtin_theme(theme)                        │         self = ResponseTextArea() │                       │
│    736 │   │   │   if theme_object is None:                                                     │        theme = 'neofusion'        │                       │
│ ❱  737 │   │   │   │   raise ThemeDoesNotExist(                                                 │ theme_object = None               │                       │
│    738 │   │   │   │   │   f"{theme!r} is not a builtin theme, or it has not been registered. " ╰───────────────────────────────────╯                       │
│    739 │   │   │   │   │   f"To use a custom theme, register it first using `register_theme`, "                                                             │
│    740 │   │   │   │   │   f"then switch to that theme by setting the `TextArea.theme` attribut                                                             │
╰─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
ThemeDoesNotExist: 'neofusion' is not a builtin theme, or it has not been registered. To use a custom theme, register it first using `register_theme`, then 
switch to that theme by setting the `TextArea.theme` attribute.
NOTE: 1 of 3 errors shown. Run with textual run --dev to see all errors.

.../themes/neofusion.yml

[darrenburns]

Thanks! Yeah, I should probably show a nicer error message in that case. Going forward, I'll try to have the syntax be customisable via this file too. For now, you can just leave `syntax blank.

[diegoulloao]

That would be nice, thank you.