This comes from a UDN report.
When launching the editor with -game, if a material contains a null expression (for instance because that material expression has been removed), the binary will crash, as GIsEditor is false, so null expressions won't take place in UMaterial::PostLoad().
We need to make sure the array of expressions is always cleaned up even when GIsEditor is false.
This occurs when launching the editor in game mode with the -game flag, and the launched level contains a material with an expression whose type was deleted.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-201121 in the post.