[macOS] Fix TextInputPlugin crash when no viewId is provided (flutter#169583)

Fixes flutter#169204

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Author: knopp

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterTextInputPlugin.mm

@@ -10,6 +10,7 @@
 #include <algorithm>
 #include <memory>
 
+#include "flutter/common/constants.h"
 #include "flutter/fml/platform/darwin/string_range_sanitization.h"
 #include "flutter/shell/platform/common/text_editing_delta.h"
 #include "flutter/shell/platform/common/text_input_model.h"
@@ -425,8 +426,12 @@ - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
       }
 
       _activeModel = std::make_unique<flutter::TextInputModel>();
-      NSNumber* viewId = config[kViewId];
-      _currentViewController = [_delegate viewControllerForIdentifier:viewId.longLongValue];
+      FlutterViewIdentifier viewId = flutter::kFlutterImplicitViewId;
+      NSObject* requestViewId = config[kViewId];
+      if ([requestViewId isKindOfClass:[NSNumber class]]) {
+        viewId = [(NSNumber*)requestViewId longLongValue];
+      }
+      _currentViewController = [_delegate viewControllerForIdentifier:viewId];
       FML_DCHECK(_currentViewController != nil);
     }
   } else if ([method isEqualToString:kShowMethod]) {

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterTextInputPluginTest.mm

@@ -60,6 +60,7 @@ - (bool)testClearClientDuringComposing;
 @interface FlutterTextInputPluginTestDelegate : NSObject <FlutterTextInputPluginDelegate> {
   id<FlutterBinaryMessenger> _binaryMessenger;
   FlutterViewController* _viewController;
+  FlutterViewController* _implicitViewController;
 }
 
 @end
@@ -80,10 +81,22 @@ - (instancetype)initWithBinaryMessenger:(id<FlutterBinaryMessenger>)messenger
   return self;
 }
 
+- (instancetype)initWithBinaryMessenger:(id<FlutterBinaryMessenger>)messenger
+                 implicitViewController:(FlutterViewController*)viewController {
+  self = [super init];
+  if (self) {
+    _binaryMessenger = messenger;
+    _implicitViewController = viewController;
+  }
+  return self;
+}
+
 - (nullable FlutterViewController*)viewControllerForIdentifier:
     (FlutterViewIdentifier)viewIdentifier {
   if (viewIdentifier == kViewId) {
     return _viewController;
+  } else if (viewIdentifier == flutter::kFlutterImplicitViewId) {
+    return _implicitViewController;
   } else {
     return nil;
   }
@@ -2394,4 +2407,32 @@ - (bool)testSelectorsNotForwardedToFrameworkIfNoClient {
   ASSERT_TRUE(plugin.clipsToBounds);
 }
 
+TEST(FlutterTextInputPluginTest, WorksWithoutViewId) {
+  id engineMock = flutter::testing::CreateMockFlutterEngine(@"");
+  id binaryMessengerMock = OCMProtocolMock(@protocol(FlutterBinaryMessenger));
+  OCMStub(  // NOLINT(google-objc-avoid-throwing-exception)
+      [engineMock binaryMessenger])
+      .andReturn(binaryMessengerMock);
+
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engineMock
+                                                                                nibName:@""
+                                                                                 bundle:nil];
+
+  FlutterTextInputPluginTestDelegate* delegate =
+      [[FlutterTextInputPluginTestDelegate alloc] initWithBinaryMessenger:binaryMessengerMock
+                                                   implicitViewController:viewController];
+
+  FlutterTextInputPlugin* plugin = [[FlutterTextInputPlugin alloc] initWithDelegate:delegate];
+
+  NSDictionary* setClientConfig = @{
+      // omit viewId
+  };
+  [plugin handleMethodCall:[FlutterMethodCall methodCallWithMethodName:@"TextInput.setClient"
+                                                             arguments:@[ @(1), setClientConfig ]]
+                    result:^(id){
+                    }];
+
+  ASSERT_TRUE(plugin.currentViewController == viewController);
+}
+
 }  // namespace flutter::testing