Fixes crashing disposed listeners on a bunch of widgets (#164)

lib/widgets/controller.dart

@@ -32,6 +32,13 @@ class QuillController extends ChangeNotifier {
   Style toggledStyle = Style();
   bool ignoreFocusOnTextChange = false;
 
+  /// Controls whether this [QuillController] instance has already been disposed
+  /// of
+  /// 
+  /// This is a safe approach to make sure that listeners don't crash when
+  /// adding, removing or listeners to this instance.
+  bool _isDisposed = false;
+
   // item1: Document state before [change].
   //
   // item2: Change delta applied to the document.
@@ -183,9 +190,31 @@ class QuillController extends ChangeNotifier {
     notifyListeners();
   }
 
+  @override
+  void addListener(VoidCallback listener) {
+    // By using `_isDisposed`, make sure that `addListener` won't be called on a
+    // disposed `ChangeListener`
+    if (!_isDisposed) {
+      super.addListener(listener);
+    }
+  }
+
+  @override
+  void removeListener(VoidCallback listener) {
+    // By using `_isDisposed`, make sure that `removeListener` won't be called
+    // on a disposed `ChangeListener`
+    if (!_isDisposed) {
+      super.removeListener(listener);
+    }
+  }
+
   @override
   void dispose() {
-    document.close();
+    if (!_isDisposed) {
+      document.close();
+    }
+
+    _isDisposed = true;
     super.dispose();
   }