Android App keeps on crashing upon new Intent

So I've got this error code: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference

The line: Intent callMode = new Intent(MainActivity.this, IncallActivity.class);

What I'm trying to do is to set a new activity when I receive a call. The following function is what should "switch" to that activity.

public void callMode() {
    Intent callMode = new Intent(MainActivity.this, IncallActivity.class);
    startActivity(callMode);
}

And it is located inside of the MainActivity class, outside of the onCreate function.
When I try to put it inside of the onCreate function (without public void callMode() of course) it works, but that's not the goal.

I'm trying to activate this new activity from another class, which listens for calls.

public class CallReceiver extends BroadcastReceiver {
    IncallActivity incallActivity = new IncallActivity();
    public MainActivity mainActivity;

    @Override
    public void onReceive(Context context, Intent intent) {
        if (intent.getStringExtra(TelephonyManager.EXTRA_STATE).equals(TelephonyManager.EXTRA_STATE_OFFHOOK)) {
            mainActivity.callMode();
        } else if (intent.getStringExtra(TelephonyManager.EXTRA_STATE).equals(TelephonyManager.EXTRA_STATE_IDLE)) {
            incallActivity.endCallMode();
        }
    }
}

So why does this error show up?

I found a way to do it.

You need to add a public static Context context to your code, and then in onCreate, do context = getBaseContext();

From there you can launch the second activity from any class you'd like.