First Change it
dialog_ratingbar = (RatingBar) findViewById(R.id.dialog_ratingbar);
to
dialog_ratingbar = (RatingBar) dialog.findViewById(R.id.dialog_ratingbar);
//And try like this to get the value of Rating Bar on On Button Click.
String answerValue =null;
Button dialogButtonOK = (Button) dialog.findViewById(R.id.dialogButtonOK);
dialog_ratingbar.setOnRatingBarChangeListener(new OnRatingBarChangeListener() {
public void onRatingChanged(RatingBar ratingBar,
float rating, boolean fromUser) {
answerValue = String.valueOf(ratingBar.getRating());// Get the Rating Here
}
});
dialogButtonOK.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
Toast.makeText(getApplicationContext(),"Your Selected Ratings : " + answerValue,Toast.LENGTH_LONG).show();
dialog.dismiss();
}
});