Available in minor release 2018.3.1122. It will also be available in the R1 2019 release.
Available in minor release 2018.2.821. It will also be available in the R3 2018 release.
Available in minor release 2018.3.1109. It will also be available in the R1 2019 release.
As a workaround you could set TextColor property of the Entry control.
Explicitly setting the Text property of the RadEntry to null results in NullReferenceException. This is not the behavior with the standard Entry control.