Fix new analyzer rule failure (flutter#67656)

Author: justinmc

packages/flutter/test/material/floating_action_button_test.dart

@@ -107,17 +107,15 @@ void main() {
     expect(find.text('Add'), findsNothing);
 
     // Test hover for tooltip.
-    TestGesture? gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
-    await gesture!.addPointer();
-    addTearDown(() => gesture?.removePointer());
+    final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
+    await gesture.addPointer();
+    addTearDown(() => gesture.removePointer());
     await gesture.moveTo(tester.getCenter(find.byType(FloatingActionButton)));
     await tester.pumpAndSettle();
 
     expect(find.text('Add'), findsOneWidget);
 
     await gesture.moveTo(Offset.zero);
-    await gesture.removePointer();
-    gesture = null;
     await tester.pumpAndSettle();
 
     expect(find.text('Add'), findsNothing);
@@ -144,18 +142,16 @@ void main() {
     expect(find.text('Add'), findsNothing);
 
     // Test hover for tooltip.
-    TestGesture? gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
-    await gesture!.addPointer();
-    addTearDown(() => gesture?.removePointer());
+    final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
+    await gesture.addPointer();
+    addTearDown(() => gesture.removePointer());
     await tester.pumpAndSettle();
     await gesture.moveTo(tester.getCenter(find.byType(FloatingActionButton)));
     await tester.pumpAndSettle();
 
     expect(find.text('Add'), findsOneWidget);
 
     await gesture.moveTo(Offset.zero);
-    await gesture.removePointer();
-    gesture = null;
     await tester.pumpAndSettle();
 
     expect(find.text('Add'), findsNothing);