Steps to reproduce:
1. Open QA attached project
2. Play the scene and notice that Reflection probe object assigns Render Texture as Cubemap
3. Select Reflection Probe object
4. Select "New Render Texture" in the project window
5. Notice that dimensions are set to "Cube"
6. Try to drag and drop "New Render Texture" on Custom Reflection Probes "Cubemap" slot
7. Notice that you are not allowed to assign Cube Render Texture

Expected result: assigning Cubemap Render Texture to the Cubemap slot of Reflection Probe should be allowed

Reproduced in: 2018.2.0a5, 2018.1.0b13, 2017.4.0f1, 2017.3.2f1, 2017.2.2p1, 2017.1.3p2

Fixed in: 2019.1.0a2