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

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

@@ -2440,6 +2440,13 @@ - (void)addToInputParentViewIfNeeded:(FlutterTextInputView*)inputView {
   if (![inputView isDescendantOfView:_inputHider]) {
     [_inputHider addSubview:inputView];
   }
+
+  if (_viewController.view == nil) {
+    // If view controller's view has detached from flutter engine, we don't add _inputHider
+    // in parent view to fallback and avoid crash.
+    // https://github.com/flutter/flutter/issues/106404.
+    return;
+  }
   UIView* parentView = self.hostView;
   if (_inputHider.superview != parentView) {
     [parentView addSubview:_inputHider];

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

@@ -166,6 +166,24 @@ - (FlutterTextRange*)getLineRangeFromTokenizer:(id<UITextInputTokenizer>)tokeniz
 
 #pragma mark - Tests
 
+- (void)testWillNotCrashWhenViewControllerIsNil {
+  FlutterEngine* flutterEngine = [[FlutterEngine alloc] init];
+  FlutterTextInputPlugin* inputPlugin =
+      [[FlutterTextInputPlugin alloc] initWithDelegate:(id<FlutterTextInputDelegate>)flutterEngine];
+  XCTAssertNil(inputPlugin.viewController);
+  FlutterMethodCall* methodCall = [FlutterMethodCall methodCallWithMethodName:@"TextInput.show"
+                                                                    arguments:nil];
+  XCTestExpectation* expectation = [[XCTestExpectation alloc] initWithDescription:@"result called"];
+
+  [inputPlugin handleMethodCall:methodCall
+                         result:^(id _Nullable result) {
+                           XCTAssertNil(result);
+                           [expectation fulfill];
+                         }];
+  XCTAssertNil(inputPlugin.activeView);
+  [self waitForExpectations:@[ expectation ] timeout:1.0];
+}
+
 - (void)testInvokeStartLiveTextInput {
   FlutterMethodCall* methodCall =
       [FlutterMethodCall methodCallWithMethodName:@"TextInput.startLiveTextInput" arguments:nil];