https://github.com/UnityLabs/mars-project/issues/602

NOTE: must be in new project that never imported the TMP Essentials yet

**Setup**
- mars packages 0.9.9
- Unity version 2019.3.0f6
- Mac OS 10.15.4

**Steps to Reproduce:**
1. Create a new proxy
1. Add a Plane Size condition
1. In simulation view, edit the plane size condition
1. When prompted to import the TMP Essentials, do so.

**Actual Results**
Console will print error :
```
Saving Prefab to immutable folder is not allowed: Packages/com.unity.labs.mars/Runtime/Prefabs/Handles/PlaneSizeHandle.prefab
UnityEditor.AssetDatabase:ImportPackage(String, Boolean)
TMPro.TMP_PackageResourceImporter:OnGUI() (at Library/PackageCache/com.unity.textmeshpro@2.0.1/Scripts/Runtime/TMP_PackageResourceImporter.cs:45)
TMPro.TMP_PackageResourceImporterWindow:OnGUI() (at Library/PackageCache/com.unity.textmeshpro@2.0.1/Scripts/Runtime/TMP_PackageResourceImporter.cs:189)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
```

**Expected Results**
Console should print no errors