fix: layout animation NPE crash (#6897)

## Summary

After update to Reanimated 3.16.x we started to have hundreds of crashes
due to this error:
```
Exception java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.lang.ref.WeakReference.get()' on a null object reference.
```
More info could be found in this issue #4472 

## Test plan

I am not able to reproduce issue locally since it's happening for some
small percentage of users. According to stack trace I am pretty sure it
will help but I am not sure if it's proper fix because I don't have
deeper understanding of surrounding code.

---------

Co-authored-by: Krzysztof Piaskowy <[email protected]>

packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/layoutReanimation/AnimationsManager.java

@@ -32,7 +32,7 @@
 import javax.annotation.Nullable;
 
 public class AnimationsManager implements ViewHierarchyObserver {
-  private WeakReference<AndroidUIScheduler> mWeakAndroidUIScheduler;
+  private WeakReference<AndroidUIScheduler> mWeakAndroidUIScheduler = new WeakReference<>(null);
   private ReactContext mContext;
   private UIManager mUIManager;
   private NativeMethodsHolder mNativeMethodsHolder;
@@ -105,6 +105,8 @@ public void onViewCreate(View view, ViewGroup parent, Snapshot after) {
     AndroidUIScheduler androidUIScheduler = mWeakAndroidUIScheduler.get();
     if (androidUIScheduler != null) {
       androidUIScheduler.triggerUI();
+    } else {
+      return;
     }
     int tag = view.getId();
     HashMap<String, Object> targetValues = after.toTargetMap();