Make Scaffold.of() more helpful when it can't find a Scaffold (#6612)

packages/flutter/lib/src/material/scaffold.dart

@@ -425,7 +425,35 @@ class Scaffold extends StatefulWidget {
   ///   );
   /// }
   /// ```
-  static ScaffoldState of(BuildContext context) => context.ancestorStateOfType(const TypeMatcher<ScaffoldState>());
+  ///
+  /// If there is no [Scaffold] in scope, then this will throw an exception.
+  /// To return null if there is no [Scaffold], then pass `nullOk: true`.
+  static ScaffoldState of(BuildContext context, { bool nullOk: false }) {
+    assert(nullOk != null);
+    assert(context != null);
+    ScaffoldState result = context.ancestorStateOfType(const TypeMatcher<ScaffoldState>());
+    if (nullOk || result != null)
+      return result;
+    throw new FlutterError(
+      'Scaffold.of() called with a context that does not contain a Scaffold.\n'
+      'No Scaffold could be found starting from the context that was passed to Scaffold.of(). '
+      'This usually happens when the context provided is from the same StatefulWidget as that '
+      'whose build function actually creates the Scaffold widget being sought.\n'
+      'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
+      'context that is "under" the Scaffold. For an example of this, please see the '
+      'documentation for Scaffold.of():\n'
+      '  https://docs.flutter.io/flutter/material/Scaffold/of.html\n'
+      'A more efficient solution is to split your build function into several widgets. This '
+      'introduces a new context from which you can obtain the Scaffold. In this solution, '
+      'you would have an outer widget that creates the Scaffold populated by instances of '
+      'your new inner widgets, and then in these inner widgets you would use Scaffold.of().\n'
+      'A less elegant but more expedient solution is assign a GlobalKey to the Scaffold, '
+      'then use the key.currentState property to obtain the ScaffoldState rather than '
+      'using the Scaffold.of() function.\n'
+      'The context used was:\n'
+      '  $context'
+    );
+  }
 
   @override
   ScaffoldState createState() => new ScaffoldState();