Do not add null task observers (#11315)

Potential fix for: https://github.com/flutter/flutter/issues/38778

fml/merged_queues_runner.cc

@@ -17,7 +17,7 @@ class MessageLoopTaskQueues::MergedQueuesRunner {
                      TaskQueueId owner,
                      MutexType type = MutexType::kTasks)
       : owner_(owner),
-        subsumed_(task_queues_._kUnmerged),
+        subsumed_(MessageLoopTaskQueues::_kUnmerged),
         task_queues_(task_queues),
         type_(type) {
     task_queues_.GetMutex(owner, type).lock();

fml/message_loop_impl.cc

@@ -64,7 +64,11 @@ void MessageLoopImpl::AddTaskObserver(intptr_t key, fml::closure callback) {
   FML_DCHECK(MessageLoop::GetCurrent().GetLoopImpl().get() == this)
       << "Message loop task observer must be added on the same thread as the "
          "loop.";
-  task_queue_->AddTaskObserver(queue_id_, key, callback);
+  if (callback != nullptr) {
+    task_queue_->AddTaskObserver(queue_id_, key, callback);
+  } else {
+    FML_LOG(ERROR) << "Tried to add a null TaskObserver.";
+  }
 }
 
 void MessageLoopImpl::RemoveTaskObserver(intptr_t key) {

fml/message_loop_task_queues.cc

@@ -125,6 +125,7 @@ size_t MessageLoopTaskQueues::GetNumPendingTasks(TaskQueueId queue_id) {
 void MessageLoopTaskQueues::AddTaskObserver(TaskQueueId queue_id,
                                             intptr_t key,
                                             fml::closure callback) {
+  FML_DCHECK(callback != nullptr) << "Observer callback must be non-null.";
   std::scoped_lock lock(GetMutex(queue_id, MutexType::kObservers));
   task_observers_[queue_id][key] = std::move(callback);
 }