[BUG] Indented text in-between list items can lead to broken drag and drop

[wwselleck]

Describe the bug
Putting indented content under a list item can lead to a broken tree if you add additional list items under that content. I'm unsure if there are other scenarios where this happens.

To Reproduce
Steps to reproduce the behavior:

# Initial state
- a<cursor>

# Type: SHIFT+ENTER
- a
  <cursor>
  
# Type: anything
- a
  sometext<cursor>
  
# Move cursor
 - a<cursor>
  sometext

# Type SHIFT+ENTER
- a
  <cursor>
  sometext

# Type HYPHEN SPACE
- a
  - 
  sometext
  
# Drag and drop is now broken for the tree

Expected behavior
Drag and drop continues to work, or some more obvious error is surfaced so the user can fix their tree

Screenshots

Environment (please complete the following information):

• OS: Desktop macOS 14.5
• Obsidian Version: 1.6.3
• Plugin Version: 4.8.1

Additional context
Error that gets logged

plugin:obsidian-outliner:1180 Uncaught TypeError: Cannot read properties of null (reading 'getListUnderLine')
    at DragAndDrop.startDragging (plugin:obsidian-outliner:1180:27)
    at DragAndDrop.handleMouseMove (plugin:obsidian-outliner:1098:22)