Crash on map change with non-null values for GPropertyColorationClass and request customizable property coloration color

This was found by a customer through a UDN, and they offered the fix for it, as well. The basic steps to reproduce the crash are as follows:

1. call GEditor->SetPropertyColorationTarget(...) somewhere in code with non-nullptr values
2. load a new map, and will see the crash.

They also have a request to add a customizable property color, which they included in the fix. The fix is on CL 26714996.

1. call GEditor->SetPropertyColorationTarget(...) somewhere in code with non-nullptr values
2. then load a new level

Expected: new level loads

Actual: Crash is caused