Fix new flutter analyze problems

Flutter analyze seems to have gotten smarter

lib/screens/githostsetup_clone_url.dart

@@ -62,6 +62,8 @@ class GitCloneUrlPageState extends State<GitCloneUrlPage> {
           if (!regExp.hasMatch(value)) {
             return "Invalid Input";
           }
+
+          return null;
         },
         focusNode: inputFormFocus,
         textInputAction: TextInputAction.done,
@@ -158,6 +160,8 @@ class GitCloneUrlKnownProviderPageState
           if (!regExp.hasMatch(value)) {
             return "Invalid Input";
           }
+
+          return null;
         },
         focusNode: inputFormFocus,
         textInputAction: TextInputAction.done,

lib/screens/githostsetup_screens.dart

@@ -286,15 +286,16 @@ class GitHostSetupScreenState extends State<GitHostSetupScreen> {
     );
 
     return WillPopScope(
-      onWillPop: () {
+      onWillPop: () async {
         if (_currentPageIndex != 0) {
           pageController.previousPage(
             duration: Duration(milliseconds: 200),
             curve: Curves.easeIn,
           );
-        } else {
+          return false;
-          Navigator.pop(context);
         }
+
+        return true;
       },
       child: scaffold,
     );

lib/screens/settings_screen.dart

@@ -52,6 +52,7 @@ class SettingsListState extends State<SettingsList> {
           if (value.isEmpty) {
             return 'Please enter a name';
           }
+          return null;
         },
         textInputAction: TextInputAction.done,
         onFieldSubmitted: saveGitAuthor,
@@ -89,6 +90,7 @@ class SettingsListState extends State<SettingsList> {
           if (!emailValid) {
             return 'Please enter a valid email';
           }
+          return null;
         },
         textInputAction: TextInputAction.done,
         onFieldSubmitted: saveGitAuthorEmail,