fix potential call on null (#14731)

* fix potential call on null

* review

packages/flutter/lib/src/cupertino/picker.dart

@@ -143,7 +143,7 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
   /// the lens and partially grayed out around it.
   Widget _buildMagnifierScreen() {
     final Color foreground = widget.backgroundColor?.withAlpha(
-      (widget.backgroundColor?.alpha * _kForegroundScreenOpacityFraction).toInt()
+      (widget.backgroundColor.alpha * _kForegroundScreenOpacityFraction).toInt()
     );
 
     return new IgnorePointer(

packages/flutter/test/widgets/form_test.dart

@@ -124,7 +124,7 @@ void main() {
     final GlobalKey<FormState> formKey = new GlobalKey<FormState>();
     final GlobalKey<FormFieldState<String>> fieldKey = new GlobalKey<FormFieldState<String>>();
     // Input 2's validator depends on a input 1's value.
-    String errorText(String input) => fieldKey.currentState.value?.toString() + '/error';
+    String errorText(String input) => '${fieldKey.currentState.value}/error';
 
     Widget builder() {
       return new Directionality(