android - NullPointerException in startIntentSenderForResult -

i want upload image googledrive, , use example github(quickstart) .i invoke method uploadfiletodrive, locates in help-class, extends mainactivity.

    void uploadfiletodrive(final bitmap bitmap) {     drive.driveapi.newcontents(mainactivity.mgoogleapiclient).setresultcallback(new resultcallback<contentsresult>() {          @override         public void onresult(contentsresult res) {              outputstream outputstream = res.getcontents().getoutputstream();                 bytearrayoutputstream bitmapstream = new bytearrayoutputstream();                 bitmap.compress(bitmap.compressformat.png, 100, bitmapstream);                 try {                     outputstream.write(bitmapstream.tobytearray());                 } catch (ioexception e1) {                     clients.prt("firstexception");                 }                 metadatachangeset metadatachangeset = new metadatachangeset.builder()                         .setmimetype("image/jpeg").settitle("android photo.png").build();                  intentsender intentsender = drive.driveapi                         .newcreatefileactivitybuilder()                         .setinitialmetadata(metadatachangeset)                         .setinitialcontents(res.getcontents())                         .build(mainactivity.mgoogleapiclient);                  try {                    startintentsenderforresult( //exception line                             intentsender, 0, null, 0, 0, 0);                 } catch (exception e) {                     e.printstacktrace();                 }             }         });   } 

but nullpointerexception:

04-22 12:22:52.548: w/system.err(2377): java.lang.nullpointerexception 04-22 12:22:52.558: w/system.err(2377): @ android.app.activity.startintentsenderforresultinner(activity.java:3499) 04-22 12:22:52.558: w/system.err(2377): @ android.app.activity.startintentsenderforresult(activity.java:3476) 04-22 12:22:52.588: w/system.err(2377): @ android.app.activity.startintentsenderforresult(activity.java:3443) 04-22 12:22:52.588: w/system.err(2377): @ com.example.mathdirectory.materials.drivehelper$2.onresult(drivehelper.java:128)

the connect googledrive correct, can image. please, help, problem?