Deleting MPC referenced in material layer cause editor crash.

1.Create a material layer and add set it to a material attribute layers node.

2.Create a material parameter collection (MPC) and use it in the material layer.

3.Create a material instance from the material.

4.Force delete the MPC and double click the material instance asset.

5.UE editor will crash without error or warning.

6.Reopen editor and open material instance again, and you will get the error info correctly.