Fix method signature

Summary:
All `NSNumber *` arguments of NativeModules need to be marked with `__nonnull`. Otherwise, when TurboModules are disabled, we run into a RedBox.

Changelog:
[iOS][Fixed] - Fix RCTDevLoadingView RedBox on Reload

Reviewed By: PeteTheHeat

Differential Revision: D20292188

fbshipit-source-id: a8a5d0b2070575d7728b6308b129196242671fe6

React/CoreModules/RCTDevLoadingView.mm

@@ -113,7 +113,7 @@ -(void)showMessage:(NSString *)message color:(UIColor *)color backgroundColor:(U
   });
 }
 
-RCT_EXPORT_METHOD(showMessage:(NSString *)message withColor:(NSNumber *)color withBackgroundColor:(NSNumber *)backgroundColor)
+RCT_EXPORT_METHOD(showMessage:(NSString *)message withColor:(NSNumber * __nonnull)color withBackgroundColor:(NSNumber * __nonnull)backgroundColor)
 {
   [self showMessage:message color:[RCTConvert UIColor:color] backgroundColor:[RCTConvert UIColor:backgroundColor]];
 }