Android :: Android - Why Application Force To Close?

I admit, I'm new at this whole Android stuff. I am trying to make an app but randomly I get Force close errors and I really don't know why. My application has many activities, none of them finish() when I start a new one. I get data from the web (via web services and direct image downloading) and I use AsyncTask a lot. Most of the time it crashes on the asynctask. Here is a sample on how I do things:

private BackTask backTask;
Activity ctx = this;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.trackslist);
backTask = new BackTask();
backTask.execute();
} protected class BackTask extends AsyncTask<Context, String, myObject> {
@Override protected myObject doInBackground(Context... params) { try{
if (hasInternet(ctx)==true) {
//access the web SERVICE here
//initialize myObject WITH result FROM the web
return myObject } else { return null }
}catch(Exception ex){ return null;
} } @Override protected void onPreExecute() { super.onPreExecute();
} @Override protected void onProgressUpdate(String... values) {
super.onProgressUpdate(values);
} @Override protected void onCancelled( {
super.onCancelled();
} @Override protected void onPostExecute( myObject result) {
super.onPostExecute(result);
if (result==null || result.isEmpty()) {
//no valid result, show a message } else {
//result valid do something with it } }
} @Override public void onPause() {
if (backTask!=null && ! backTask.isCancelled()) {
backTask.cancel(true);
} super.onPause();
} public void btnStartOnClick(View target) {
Intent intent = new Intent(this, MyNewActivity.class);
startActivity(intent);
}
When the activity gets onPause() the task is being canceled. I am not sure what happens during the try/catch if a error appears, from what I've did, it should return null, but I think here I miss something. As I said before, randomly I get a force close even if I am on another Activity. This is really frustrating as I can't offer a app that has this behavior.