If you have more than one variable curve in an animation and they are different types (either morph or material) the editor will crash if you attempt to change all of them to one or the other at the same time.
The crash wont occur if the variable curves are specified as both types. There must be at least one that is exclusive to morph and one exclusive to material.
All curves change to the specified type.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-37155 in the post.