Fixes crashing disposed listeners on a bunch of widgets

[matuella]

When a QuillController is disposed and then a new instance is added to any QuillToolbar or QuillEditor, a bunch of listeners fail to remove because it has already been disposed of, mostly because of this specific call:

oldWidget.controller.removeListener(_didChangeEditingValue);

I have created a gist for anyone interested in checking out a really easy reproduction.

Full error when tapping the error mentioned

════════ Exception caught by widgets library ═══════════════════════════════════
The following assertion was thrown building Visibility(visible, maintainState, maintainAnimation, maintainSize, maintainSemantics, maintainInteractivity):
A QuillController was used after being disposed.

Once you have called dispose() on a QuillController, it can no longer be used.
The relevant error-causing widget was
QuillToolbar
lib/main_test.dart:39
When the exception was thrown, this was the stack
#0      ChangeNotifier._debugAssertNotDisposed.<anonymous closure>
package:flutter/…/foundation/change_notifier.dart:117
#1      ChangeNotifier._debugAssertNotDisposed
package:flutter/…/foundation/change_notifier.dart:123
#2      ChangeNotifier.removeListener
package:flutter/…/foundation/change_notifier.dart:195
#3      _ToggleStyleButtonState.didUpdateWidget
package:flutter_quill/widgets/toolbar.dart:236
#4      StatefulElement.update
package:flutter/…/widgets/framework.dart:4682
...
════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by widgets library ═══════════════════════════════════
A QuillController was used after being disposed.
The relevant error-causing widget was
QuillToolbar
lib/main_test.dart:39
════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by widgets library ═══════════════════════════════════
A QuillController was used after being disposed.
The relevant error-causing widget was
QuillToolbar
lib/main_test.dart:39
════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by widgets library ═══════════════════════════════════
A QuillController was used after being disposed.
The relevant error-causing widget was
QuillToolbar
lib/main_test.dart:39
════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by widgets library ═══════════════════════════════════
A QuillController was used after being disposed.
The relevant error-causing widget was
QuillToolbar
lib/main_test.dart:39
════════════════════════════════════════════════════════════════════════════════

[matuella]

Hey @singerdmx, what do you think about this?

Also, what do you think about adding at least 1 official contributor's PR approval to merge in the main repository? I think this is just to make sure that we have a double check on what is being merged.

[singerdmx]

Not sure how to set that up ...

[matuella]

https://docs.github.com/en/github/administering-a-repository/managing-a-branch-protection-rule

Here is a quick and easy to follow tutorial!

This is just to make sure that when you are adding new collaborators (like you have done with me), there isn't any way for any of us breaking stuff and adding malicious stuff.

[singerdmx]

As long as it is easy to revert wrong commit
I have required linear history - hate messy tree