Android : Issue With N-Puzzle Android Application

I'm developing a simple N-Puzzle Android application to become more comfortable with the Android environment (I'm a beginner Android Developer). The issue I've ran into is that I can't seem to get the grid I've created to display properly. I'm at my wits end with this issue, so any help is appreciated. I can post more of the code if needed. When I attempt to run my app, I get a NullPointerException caused by the line I've commented below. I'm not sure what's causing the exception, so any insight is much appreciated.

import android.content.SharedPreferences.Editor;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewGroup.LayoutParams;
import android.widget.RelativeLayout;

public abstract class Activity extends android.app.Activity{

private Object contentView;
private boolean finished;
private int frameViewCount;

public Activity()
{

}

public Activity(View view)
{
    contentView = view;
}

public Activity(int viewResource)
{
    contentView = viewResource;
}

@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    Application app = getApplicationContext();
    app.onActivityCreate(this);
    finished = false;
    super.setContentView(R.layout.frame);
    ViewGroup frame = (ViewGroup)findViewById(R.id.viewgroup_app_frame);
    frameViewCount = frame.getChildCount();
    forceSetContentView((Integer)contentView); //this is where my issue is
}

@Override
public void onDestroy()
{
    super.onDestroy();
    getApplicationContext().onActivityDestroy(this);
}

@Override
public void setContentView(int resID)
{
    if(contentView == null || !contentView.equals(resID))
    {
        forceSetContentView(resID);
    }
}

 @Override
 public void setContentView(View view, LayoutParams params)
 {
  ViewGroup frame = (ViewGroup)findViewById(R.id.viewgroup_app_frame);
  if (frame.getChildAt(frameViewCount) != null)
   frame.removeViewAt(frameViewCount);
  addContentToFrame(frame, view, params);
 }

 @Override
 public void setContentView(View view)
 {
  if (view != contentView)
   forceSetContentView(view);
 }

@Override
public void onPause()
{
    super.onPause();
    try{
        Editor settings = getSharedPreferences("Activity.prefs", MODE_PRIVATE).edit();
        if(finished)
            settings.remove("LastActivity");
        else
            settings.putString("LastActivity", this.getClass().getName());
        settings.commit();
    }catch(Exception e)
    {
        Log.w("Activity.java", "Failed to update preferences");
    }
}

@Override
public Application getApplicationContext()
{
    return (Application)super.getApplication();
}


public void forceSetContentView(int resID)
{
    ViewGroup frame = (ViewGroup)findViewById(R.id.viewgroup_app_frame);
    if(frame.getChildAt(frameViewCount) != null)
    {
        frame.removeViewAt(frameViewCount);
    }
    View view = View.inflate(this,  resID, null);
    addContentToFrame(frame, view, null);
}

 private void forceSetContentView(View view)
 {
  ViewGroup frame = (ViewGroup)findViewById(R.id.viewgroup_app_frame);
  if (frame.getChildAt(frameViewCount) != null)
   frame.removeViewAt(frameViewCount);
  if (null != view)
   addContentToFrame(frame, view, null);
 }

public void addContentToFrame(ViewGroup frame, View view, LayoutParams params)
{
    RelativeLayout.LayoutParams frameParams;
      if (null == params)
          frameParams = new RelativeLayout.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
      else if (params instanceof RelativeLayout.LayoutParams)
          frameParams = (RelativeLayout.LayoutParams)params;
      else
          frameParams = new RelativeLayout.LayoutParams(params);
      frame.addView(view, frameViewCount, frameParams);
}

}