[iOS][Text Input] Avoid crash when its UIViewController.view is nil.

shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -2081,6 +2081,11 @@ - (void)removeEnableFlutterTextInputViewAccessibilityTimer {
 }
 
 - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
+  if (_viewController.view == nil) {
+    result(nil);
+    return;
+  }
+
   NSString* method = call.method;
   id args = call.arguments;
   if ([method isEqualToString:kShowMethod]) {

shell/platform/darwin/ios/framework/Source/FlutterTextInputPluginTest.mm

@@ -166,6 +166,20 @@ - (FlutterTextRange*)getLineRangeFromTokenizer:(id<UITextInputTokenizer>)tokeniz
 
 #pragma mark - Tests
 
+- (void)testTextInputPluginWillNotHandleMethodWhenViewControllerIsNil {
+  FlutterEngine* flutterEngine = [[FlutterEngine alloc] init];
+  FlutterTextInputPlugin* inputPlugin =
+      [[FlutterTextInputPlugin alloc] initWithDelegate:(id<FlutterTextInputDelegate>)flutterEngine];
+  XCTAssertNil(inputPlugin.viewController);
+  FlutterMethodCall* methodCall = [FlutterMethodCall methodCallWithMethodName:@"TextInput.show"
+                                                                    arguments:nil];
+  [inputPlugin handleMethodCall:methodCall
+                         result:^(id _Nullable result){
+
+                         }];
+  XCTAssertNil(inputPlugin.activeView);
+}
+
 - (void)testInvokeStartLiveTextInput {
   FlutterMethodCall* methodCall =
       [FlutterMethodCall methodCallWithMethodName:@"TextInput.startLiveTextInput" arguments:nil];