Tentative fix for NPE JavaTimerManager$IdleCallbackRunnable.cancel (#44852)

Summary:
Pull Request resolved: #44852

This attempts to fix #44842 by capturing the accessed field in a new variable.
We don't have a way to reproduce this & this is a best guess fix.

Changelog:
[Android] [Fixed] - Tentative fix for NPE `JavaTimerManager$IdleCallbackRunnable.cancel`

Reviewed By: javache

Differential Revision: D58356826

fbshipit-source-id: d016df9a52f81a8d645a0a100c6bc6111841e24e

Author: cortinico

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/modules/core/JavaTimerManager.java

@@ -99,8 +99,11 @@ public void doFrame(long frameTimeNanos) {
       }
 
       // If the JS thread is busy for multiple frames we cancel any other pending runnable.
-      if (mCurrentIdleCallbackRunnable != null) {
-        mCurrentIdleCallbackRunnable.cancel();
+      // We also capture the idleCallbackRunnable to tentatively fix:
+      // https://github.com/facebook/react-native/issues/44842
+      IdleCallbackRunnable idleCallbackRunnable = mCurrentIdleCallbackRunnable;
+      if (idleCallbackRunnable != null) {
+        idleCallbackRunnable.cancel();
       }
 
       mCurrentIdleCallbackRunnable = new IdleCallbackRunnable(frameTimeNanos);