Started allowing setting message handlers to nil for dead engines

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

@@ -678,9 +678,12 @@ - (void)sendOnChannel:(NSString*)channel
 - (void)setMessageHandlerOnChannel:(NSString*)channel
               binaryMessageHandler:(FlutterBinaryMessageHandler)handler {
   NSParameterAssert(channel);
-  NSAssert(_shell && _shell->IsSetup(),
-           @"Setting a message handler before the FlutterEngine has been run.");
-  self.iosPlatformView->GetPlatformMessageRouter().SetMessageHandler(channel.UTF8String, handler);
+  if (_shell && _shell->IsSetup()) {
+    self.iosPlatformView->GetPlatformMessageRouter().SetMessageHandler(channel.UTF8String, handler);
+  } else {
+    NSAssert(!handler, @"Setting a message handler before the FlutterEngine has been run.");
+    // Setting a handler to nil for a not setup channel is a noop.
+  }
 }
 
 #pragma mark - FlutterTextureRegistry

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

@@ -47,6 +47,14 @@ - (void)testSetMessageHandlerBeforeRun {
             }]);
 }
 
+- (void)testNilSetMessageHandlerBeforeRun {
+  id project = OCMClassMock([FlutterDartProject class]);
+  FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"foobar" project:project];
+  XCTAssertNotNil(engine);
+  XCTAssertNoThrow([engine.binaryMessenger setMessageHandlerOnChannel:@"foo"
+                                                 binaryMessageHandler:nil]);
+}
+
 - (void)testNotifyPluginOfDealloc {
   id plugin = OCMProtocolMock(@protocol(FlutterPlugin));
   OCMStub([plugin detachFromEngineForRegistrar:[OCMArg any]]);