I would suggest, one should not make object of an Activity type class.
MainActivity mActivity = new MainActivity(); // BIG NO TO THIS.
All Activities in Android must go through the Activity lifecycle so that they have a valid context attached to them.
By treating an Activity as a normal Java class, you end up with a null context. As most methods in an Activity are called on its Context, you will get a null pointer exception, which is why your app crashes.
Instead, move all such methods which need to be called from other classes into a Utility class which accepts a valid context in its constructor, and then use that context in the methods to do the work.

You cannot just create objects of Activities by using:
**MyActivity activity = new MyActivity();**
as you would with normal Java classes. All Activities in Android must go through the Activity lifecycle so that they have a valid context attached to them.
By treating an Activity as a normal Java class, you end up with a null context. As most methods in an Activity are called on its Context, you will get a null pointer exception, which is why your app crashes.
Instead, move all such methods which need to be called from other classes into a Utility class which accepts a valid context in its constructor, and then use that context in the methods to do the work.