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Messages sent to the embedder host may be one-way messages with no response handler. If the host calls FlutterEngineSendPlatformMessageResponse on a one-way message, then just delete the message response handle. Also update the documentation to indicate that FlutterEngineSendPlatformMessageResponse must be called for all messages. Previously the docs implied that some FlutterPlatformMessage objects may have a null response_handle. The embedder will now set a response_handle for every message (even if the sender does not expect a response).