Make requesting the trait collection synchronous

Author: alanjhughes

packages/react-native/React/CoreModules/RCTAppearance.mm

@@ -39,10 +39,15 @@ void RCTOverrideAppearancePreference(NSString *const colorSchemeOverride)
 static UITraitCollection *getKeyWindowTraitCollection()
 {
   __block UITraitCollection *traitCollection = nil;
-  RCTExecuteOnMainQueue(^{
-    traitCollection = RCTSharedApplication().delegate.window.traitCollection;
-  });
-  return traitCollection;
+  if (RCTIsMainQueue()) {
+    return RCTSharedApplication().delegate.window.traitCollection;
+  } else {
+    __block UITraitCollection* traitCollection = nil;
+    dispatch_sync(dispatch_get_main_queue(), ^{
+      traitCollection = RCTSharedApplication().delegate.window.traitCollection;
+    });
+    return traitCollection;
+  }
 }
 
 NSString *RCTColorSchemePreference(UITraitCollection *traitCollection)
@@ -68,9 +73,6 @@ void RCTOverrideAppearancePreference(NSString *const colorSchemeOverride)
 
   traitCollection = traitCollection ?: getKeyWindowTraitCollection();
   return appearances[@(traitCollection.userInterfaceStyle)] ?: RCTAppearanceColorSchemeLight;
-
-  // Default to light on older OS version - same behavior as Android.
-  return RCTAppearanceColorSchemeLight;
 }
 
 @interface RCTAppearance () <NativeAppearanceSpec>