findFragmentById return null

Question

When i call findFragmentById() with the id of my fragment, it return null. Let me show you the code.

The activity_main.xml

Answer 1

just found out my mistake.

In my MainActivity.java i was importing android.support.v4.app.Fragment; and in my LoginFragment.java i was importing android.app.Fragment;. I changed it to the same thing and fm.findFragmentById(R.id.login_fragment) now return the right fragment.