dispose of controllers in guest book state

Author: ericwindmill

firebase-get-to-know-flutter/codelab_rebuild.yaml

@@ -2166,6 +2166,12 @@ steps:
                 ),
               );
             }
+          
+            @override
+            void dispose() {
+              _controller.dispose();
+              super.dispose();
+            }
           }
       - name: Patch lib/home_page.dart
         path: gtk_flutter/lib/home_page.dart

firebase-get-to-know-flutter/step_06/lib/guest_book.dart

@@ -64,4 +64,10 @@ class _GuestBookState extends State<GuestBook> {
       ),
     );
   }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
 }

firebase-get-to-know-flutter/step_07/lib/guest_book.dart

@@ -79,4 +79,10 @@ class _GuestBookState extends State<GuestBook> {
       ],
     );
   }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
 }

firebase-get-to-know-flutter/step_09/lib/guest_book.dart

@@ -79,4 +79,10 @@ class _GuestBookState extends State<GuestBook> {
       ],
     );
   }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
 }