When an assertion in C++ failes during PIE or during the customization of an Actor, the whole Engine (meaning the Editor, too) crashes. It probably shouldn’t crash the whole Edtior, since for fixing the check it’s probably required, that your are in the Editor to fix it. (E.g. by modifying a dependent UASSET or something smiliar.)
This also happens in the latest 4.13 Preview
The purpose of an Assert is to stop the program. This is shown in two different ways in Unreal. The first is if you are running the editor from Visual Studio, a break point will be created at the Assert and a call stack will be created. This often results in you having to restart the editor via Visual Studio because again, the purpose of an Assert is to create your own sort of error where the program needs to stop because something is wrong.
The second way Unreal handles an assert is when you are not running via Visual Studio. This is through the Unreal Crash window. This will stop the program and show you the call stack, including the Assert that was called.
Thank you for submitting a bug report, however at this time we believe that the issue you are describing is not actually a bug with the Unreal Engine, and so we are not able to take any further action on this.