diff --git a/spyder_notebook/widgets/client.py b/spyder_notebook/widgets/client.py
index 7aa0259e..0dae552c 100644
--- a/spyder_notebook/widgets/client.py
+++ b/spyder_notebook/widgets/client.py
@@ -153,6 +153,11 @@ class NotebookClient(QWidget):
 
     This is a widget composed of a NotebookWidget and a find dialog to
     render notebooks.
+
+    Attributes
+    ----------
+    server_url : str or None
+        URL to send requests to; set by register().
     """
 
     def __init__(self, parent, filename, actions=None, ini_message=None):
@@ -229,7 +234,7 @@ def register(self, server_info):
         self.file_url = self.add_token(url)
 
     def go_to(self, url_or_text):
-        """Go to page utl."""
+        """Go to page URL."""
         if isinstance(url_or_text, str):
             url = QUrl(url_or_text)
         else:
@@ -254,22 +259,29 @@ def get_short_name(self):
 
     def save(self):
         """
-        Save current notebook.
-
-        This function saves the current notebook by simulating a click on the
-        Save button in the notebook. The Save button is found by selecting the
-        first element of class `jp-ToolbarButtonComponent` whose `title`
-        attribute begins with the string "Save".
+        Save current notebook asynchronously.
 
-        NB: I am not sure whether the save is performed before the function
-        returns.
+        This function simulates a click on the Save button in the notebook
+        which will save the current notebook (but the function will return
+        before). The Save button is found by selecting the first element of
+        class `jp-ToolbarButtonComponent` whose `title` attribute begins with
+        the string "Save".
         """
         self.notebookwidget.mousedown(
             '.jp-ToolbarButtonComponent[title^="Save"]')
 
     def get_session_url(self):
-        """Get the kernel sessions url of the client."""
-        return self.add_token(url_path_join(self.server_url, 'api/sessions'))
+        """
+        Get the kernel sessions URL of the client.
+
+        Return a str with the URL or None, if no server is associated to
+        the client.
+        """
+        if self.server_url:
+            session_url = url_path_join(self.server_url, 'api/sessions')
+            return self.add_token(session_url)
+        else:
+            return None
 
     def get_kernel_id(self):
         """
@@ -279,6 +291,9 @@ def get_kernel_id(self):
         box and return None.
         """
         sessions_url = self.get_session_url()
+        if not sessions_url:
+            return None
+
         try:
             sessions_response = requests.get(sessions_url)
         except requests.exceptions.RequestException as exception:
diff --git a/spyder_notebook/widgets/tests/test_client.py b/spyder_notebook/widgets/tests/test_client.py
index 624ed692..de4f8c13 100644
--- a/spyder_notebook/widgets/tests/test_client.py
+++ b/spyder_notebook/widgets/tests/test_client.py
@@ -18,10 +18,11 @@ class MockPlugin(QWidget):
     def get_plugin_actions(self):
         return []
 
+
 @pytest.fixture
-def plugin(qtbot):
+def plugin_without_server(qtbot):
     """
-    Construct mock plugin with NotebookClient for use in tests.
+    Construct mock plugin with NotebookClient but no notebook server.
 
     Use `plugin.client` to access the client.
     """
@@ -29,11 +30,21 @@ def plugin(qtbot):
     qtbot.addWidget(plugin)
     client = NotebookClient(plugin, '/path/notebooks/ham.ipynb')
     plugin.client = client
+    return plugin
+
+
+@pytest.fixture
+def plugin(plugin_without_server):
+    """
+    Construct mock plugin with NotebookClient with server registered.
+
+    Use `plugin.client` to access the client.
+    """
     server_info = {'notebook_dir': '/path/notebooks',
                    'url': 'fake_url',
                    'token': 'fake_token'}
-    client.register(server_info)
-    return plugin
+    plugin_without_server.client.register(server_info)
+    return plugin_without_server
 
 
 def test_notebookclient_get_kernel_id(plugin, mocker):
@@ -48,6 +59,17 @@ def test_notebookclient_get_kernel_id(plugin, mocker):
     assert kernel_id == '42'
 
 
+def test_notebookclient_get_kernel_id_without_server(
+        plugin_without_server, mocker):
+    """Test NotebookClient.get_kernel_id() if client has no server."""
+    mock_get = mocker.patch('requests.get')
+
+    kernel_id = plugin_without_server.client.get_kernel_id()
+
+    assert kernel_id is None
+    mock_get.assert_not_called()
+
+
 def test_notebookclient_get_kernel_id_with_fields_missing(plugin, mocker):
     """Test NotebookClient.get_kernel_id() if response has fields missing."""
     response = mocker.Mock()