Support removing absent tools from ToolContainerBase.

ToolManager.remove_tool calls ToolContainerBase.remove_toolitem
(via a tool_removed_event) and cannot know (due to the lack of API)
whether the tool is actually on the container (it can also be a
keybind-only tool not associated with any entry on the toolbar), so
ToolContainerBase.remove_toolitem should work (as a no-op) even for
tools not present on the container.

lib/matplotlib/backend_bases.py

@@ -3459,6 +3459,10 @@ def remove_toolitem(self, name):
         backend-specific code to remove an element from the toolbar; it is
         called when `.ToolManager` emits a `tool_removed_event`.
 
+        Because some tools are present only on the `.ToolManager` but not on
+        the `ToolContainer`, this method must be a no-op when called on a tool
+        absent from the container.
+
         .. warning::
             This is part of the backend implementation and should
             not be called by end-users.  They should instead call

lib/matplotlib/backends/_backend_tk.py

@@ -1011,9 +1011,8 @@ def toggle_toolitem(self, name, toggled):
                 toolitem.deselect()
 
     def remove_toolitem(self, name):
-        for toolitem in self._toolitems[name]:
+        for toolitem in self._toolitems.pop(name, []):
             toolitem.pack_forget()
-        del self._toolitems[name]
 
     def set_message(self, s):
         self._message.set(s)

lib/matplotlib/backends/backend_gtk3.py

@@ -446,15 +446,10 @@ def toggle_toolitem(self, name, toggled):
             toolitem.handler_unblock(signal)
 
     def remove_toolitem(self, name):
-        if name not in self._toolitems:
-            self.toolmanager.message_event(f'{name} not in toolbar', self)
-            return
-
-        for group in self._groups:
-            for toolitem, _signal in self._toolitems[name]:
+        for toolitem, _signal in self._toolitems.pop(name, []):
+            for group in self._groups:
                 if toolitem in self._groups[group]:
                     self._groups[group].remove(toolitem)
-        del self._toolitems[name]
 
     def _add_separator(self):
         sep = Gtk.Separator()

lib/matplotlib/backends/backend_gtk4.py

@@ -475,15 +475,10 @@ def toggle_toolitem(self, name, toggled):
             toolitem.handler_unblock(signal)
 
     def remove_toolitem(self, name):
-        if name not in self._toolitems:
-            self.toolmanager.message_event(f'{name} not in toolbar', self)
-            return
-
-        for group in self._groups:
-            for toolitem, _signal in self._toolitems[name]:
+        for toolitem, _signal in self._toolitems.pop(name, []):
+            for group in self._groups:
                 if toolitem in self._groups[group]:
                     self._groups[group].remove(toolitem)
-        del self._toolitems[name]
 
     def _add_separator(self):
         sep = Gtk.Separator()

lib/matplotlib/backends/backend_qt.py

@@ -1007,9 +1007,8 @@ def toggle_toolitem(self, name, toggled):
             button.toggled.connect(handler)
 
     def remove_toolitem(self, name):
-        for button, handler in self._toolitems[name]:
+        for button, handler in self._toolitems.pop(name, []):
             button.setParent(None)
-        del self._toolitems[name]
 
     def set_message(self, s):
         self.widgetForAction(self._message_action).setText(s)

lib/matplotlib/backends/backend_wx.py

@@ -1257,9 +1257,8 @@ def toggle_toolitem(self, name, toggled):
         self.Refresh()
 
     def remove_toolitem(self, name):
-        for tool, handler in self._toolitems[name]:
+        for tool, handler in self._toolitems.pop(name, []):
             self.DeleteTool(tool.Id)
-        del self._toolitems[name]
 
     def set_message(self, s):
         self._label_text.SetLabel(s)