audiobook: null-check MediaPlayer

Author: teleshoes

android/src/org/coolreader/tts/TTSControlService.java

@@ -1308,12 +1308,12 @@ public void work() {
           SentenceInfo nextSentenceInfo = sentenceInfo.nextSentence;
           if(sentenceInfo.audioFile == TTSControlService.this.audioFile){
             if(nextSentenceInfo.isFirstSentenceInAudioFile){
-              if(mState == State.PLAYING && !mMediaPlayer.isPlaying()){
+              if(mState == State.PLAYING && (mMediaPlayer == null || !mMediaPlayer.isPlaying())){
                 //this is the last sentence in the file, and the media player ended
                 isAfterSentence = true;
               }
             }else{
-              if(mMediaPlayer.isPlaying()){
+              if(mMediaPlayer != null && mMediaPlayer.isPlaying()){
                 double curPos = mMediaPlayer.getCurrentPosition() / 1000.0;
                 if(curPos >= nextSentenceInfo.startTime){
                   isAfterSentence = true;