fix(android): random android crash (TheWidlarzGroup#3777)

* perf: ensure we do not provide callback to native if no callback provided from app

* chore: rework bufferConfig to make it more generic and reduce ReactExoplayerView code size

* chore: improve issue template

* fix(android): avoid video view flickering at playback startup

* fix(android): fix random crash

android/src/main/java/com/brentvatne/exoplayer/ReactExoplayerView.java

@@ -1707,8 +1707,10 @@ public void setSrc(final Uri uri, final long startPositionMs, final long cropSta
 
     public void clearSrc() {
         if (srcUri != null) {
-            player.stop();
-            player.clearMediaItems();
+            if (player != null) {
+                player.stop();
+                player.clearMediaItems();
+            }
             this.srcUri = null;
             this.startPositionMs = -1;
             this.cropStartMs = -1;