fix crash when closing overlay

Author: serhiisdev

android/src/main/java/flutter/overlay/window/flutter_overlay_window/OverlayService.java

@@ -364,6 +364,7 @@ public void run() {
             mAnimationHandler.post(() -> {
                 params.x = (2 * (params.x - mDestX)) / 3 + mDestX;
                 params.y = (2 * (params.y - mDestY)) / 3 + mDestY;
+                if(windowManager == null) return;
                 windowManager.updateViewLayout(flutterView, params);
                 if (Math.abs(params.x - mDestX) < 2 && Math.abs(params.y - mDestY) < 2) {
                     TrayAnimationTimerTask.this.cancel();