When there is a custom Cubemap asset inspector in the project or in a package, the Reflection Probe preview in the inspector throws errors (see gif)

```
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ReflectionProbeEditor.OnPreviewSettings () (at <830a4c44422a4fb282f7e1f575d71e27>:0)
```

Here's the simple code snippet that causes the error:
```
using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(Cubemap))]
public class NewBehaviourScript : Editor {}
```

Repro steps:
- Open a Unity project with either builtin RP or Universal RP
- Create a Reflection Probe and Bake it
- Click on the collapsed preview of the reflection probe at the bottom to show it
- Observe that there are errors in the console.

Repro in Unity 2020.3.8f1 with Built-in RP and Universal RP.