I had the same exception and I tried many snippet that I found here on this stackoverflow discussion, but no snippets worked for me.
But I was able to resolve all issues, I will share with you the solutions :
In a First part : I tried to show a DialogFragment on a Activity but from another java class. Then by checking the attribute of that instance, I found that was an old instance of the Activity, it was not the current running Activity. [More precisly I was using socket.io, and I forgot to do a socket.off("example",example) ... so it attached to an old instance of the activity. ]
In a Second part : I was trying to show a DialogFragment in a Activity when I come back to it with an intent, but when I checked my logs, I saw that when it tried to show the fragment the activity was still not in onStart method, so it crashed the app because it didn't find the Activity class to show the fragment on it.
Some tips : check with some attributes if you are not using an old instance of your activity with which one you are trying to show your fragment, or check your activity lifecycle before showing your fragment and be sure you are in onStart or onResume before showing it.
I hope those explanations will help you.