I have the released version of UE4.8 and I have found that using a Media Framework component crashes the editor or game when set to stream from memory. In order to get the editor or game to start again I have to manually go into the contents folder and delete the created textures and Media Framework instance and let the editor fail to load them.
To reproduce:
1 - Make a material from a video using Media Framework.
2 - Set the Media Framework instance to stream from memory, not from URL.
3 - Try to start the game outside of the editor(e.g. VR preview, separate editor window).
4 - Close the editor with an asset using the material.
5 - Try to start the editor again.
It will give an memory access error because it tried to read the location 0xFFFFFFFF.