Migrated docs to nnbd (#72305)

Simply part of the movement to get our docs aligned with NNBD.

packages/flutter/lib/src/widgets/interactive_viewer.dart

@@ -34,7 +34,7 @@ import 'ticker_provider.dart';
 ///   * The [Flutter Gallery's transformations demo](https://github.com/flutter/gallery/blob/master/lib/demos/reference/transformations_demo.dart),
 ///     which includes the use of InteractiveViewer.
 ///
-/// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+/// {@tool dartpad --template=stateless_widget_scaffold}
 /// This example shows a simple Container that can be panned and zoomed.
 ///
 /// ```dart
@@ -151,7 +151,7 @@ class InteractiveViewer extends StatefulWidget {
   ///
   /// Defaults to true.
   ///
-  /// {@tool dartpad --template=stateless_widget_scaffold_no_null_safety}
+  /// {@tool dartpad --template=stateless_widget_scaffold}
   /// This example shows how to create a pannable table. Because the table is
   /// larger than the entire screen, setting `constrained` to false is necessary
   /// to allow it to be drawn to its full size. The parts of the table that
@@ -305,19 +305,19 @@ class InteractiveViewer extends StatefulWidget {
   /// listeners are notified. If the value is set, InteractiveViewer will update
   /// to respect the new value.
   ///
-  /// {@tool dartpad --template=stateful_widget_material_ticker_no_null_safety}
+  /// {@tool dartpad --template=stateful_widget_material_ticker}
   /// This example shows how transformationController can be used to animate the
   /// transformation back to its starting position.
   ///
   /// ```dart
   /// final TransformationController _transformationController = TransformationController();
-  /// Animation<Matrix4> _animationReset;
-  /// AnimationController _controllerReset;
+  /// Animation<Matrix4>? _animationReset;
+  /// late final AnimationController _controllerReset;
   ///
   /// void _onAnimateReset() {
-  ///   _transformationController.value = _animationReset.value;
+  ///   _transformationController.value = _animationReset!.value;
   ///   if (!_controllerReset.isAnimating) {
-  ///     _animationReset?.removeListener(_onAnimateReset);
+  ///     _animationReset!.removeListener(_onAnimateReset);
   ///     _animationReset = null;
   ///     _controllerReset.reset();
   ///   }
@@ -329,7 +329,7 @@ class InteractiveViewer extends StatefulWidget {
   ///     begin: _transformationController.value,
   ///     end: Matrix4.identity(),
   ///   ).animate(_controllerReset);
-  ///   _animationReset.addListener(_onAnimateReset);
+  ///   _animationReset!.addListener(_onAnimateReset);
   ///   _controllerReset.forward();
   /// }
   ///