iOS: Avoid double reload event when reloading JS

Summary:
@public
The bridge may send the reload event twice when reloading JS in some rare condition. The 2nd one was actually when the "new" bridge wasn't valid yet, so log an error if that happens. This may happen if the connection to Metro server gets into a weird state. Restarting Metro would solve the problem.

The issue was that `RCTBridgeWillReloadNotification` fired twice for each reload with metro connected.

Reviewed By: mmmulani

Differential Revision: D8509448

fbshipit-source-id: 647fb98c0a97503409a10451b8f152145947ae20

React/CxxBridge/RCTCxxBridge.mm

@@ -876,6 +876,9 @@ - (void)setUp {}
 
 - (void)reload
 {
+  if (!_valid) {
+    RCTLogError(@"Attempting to reload bridge before it's valid: %@. Try restarting the development server if connected.", self);
+  }
   [_parentBridge reload];
 }
 

React/Fabric/RCTSurfacePresenter.mm

@@ -61,6 +61,11 @@ - (instancetype)initWithBridge:(RCTBridge *)bridge
   return self;
 }
 
+- (void)dealloc
+{
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+}
+
 - (void)schedulerDidComputeMutationInstructions:(facebook::react::TreeMutationInstructionList)instructions rootTag:(ReactTag)rootTag
 {
   [_mountingManager mutateComponentViewTreeWithMutationInstructions:instructions

React/Fabric/Surface/RCTFabricSurface.mm

@@ -90,6 +90,8 @@ - (instancetype)initWithSurfacePresenter:(RCTSurfacePresenter *)surfacePresenter
 
 - (void)dealloc
 {
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+
   [self _stop];
 }