Keep the existing observer when there is no key window

RCTKeyWindow() can return nil (backgrounded app, scene transition). Dropping the
observer in that case would leave the module without one until another trigger
happens to re-attach it.

Author: HugoGresse

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

@@ -154,8 +154,10 @@ - (void)_observeKeyWindowIfNeeded
   // The key window is looked up again instead of trusting the one captured in -init: when the host
   // app creates its window after this module is built, that capture is nil, the KVO below is never
   // installed, and app activation is left as the only trigger for a dimensions update.
+  // A nil key window means there is nothing to observe yet (backgrounded app, scene transition);
+  // keeping the current observer is better than dropping it until the next trigger comes along.
   UIWindow *keyWindow = RCTKeyWindow();
-  if (keyWindow == _applicationWindow) {
+  if (keyWindow == nil || keyWindow == _applicationWindow) {
     return;
   }