Fix Undo throw Offset cannot be negative

lib/widgets/controller.dart

@@ -44,15 +44,25 @@ class QuillController extends ChangeNotifier {
 
   void undo() {
     if (document.undo()) {
+      if (this.selection.extentOffset >= document.length) {
+        updateSelection(TextSelection.collapsed(offset: document.length),
+            ChangeSource.LOCAL);
+      } else {
         notifyListeners();
       }
     }
+  }
 
   void redo() {
     if (document.redo()) {
+      if (this.selection.extentOffset >= document.length) {
+        updateSelection(TextSelection.collapsed(offset: document.length),
+            ChangeSource.LOCAL);
+      } else {
         notifyListeners();
       }
     }
+  }
 
   get hasUndo => document.hasUndo;