Android source code from which I come, because there I search for this error message:
http://crypto.nknu.edu.tw/AOSP/android-8.0.0_r4/frameworks/support/fragment/java/android/support/v4/app/Fragment.java

MainActivity}: java.lang.IllegalStateException: Fragment has not been attached yet.
Really helpfull exception...

Fragment.java:
void instantiateChildFragmentManager() {
if (mHost == null) {
throw new IllegalStateException("Fragment has not been attached yet.");
}
mChildFragmentManager = new FragmentManagerImpl();
mChildFragmentManager.attachController(mHost, new FragmentContainer() {
@Override
@Nullable
public View onFindViewById(int id) {
if (mView == null) {
throw new IllegalStateException("Fragment does not have a view");
}
return mView.findViewById(id);
}

@Override
public boolean onHasView() {
return (mView != null);
}

@Override
public Fragment instantiate(Context context, String className, Bundle arguments) {
return mHost.instantiate(context, className, arguments);
}
}, this);
}

So please just add stupid name of fragment to log like in my gist:
https://gist.github.com/mtrakal/e418a7b15a9d16dcd55a6a66ea62a465/revisions