How to reproduce:
1. Create a new project
2. Window > Package Manager >ProBuilder > Install
3. Tools > ProBuilder > ProBuilder Window
4. Create a New Poly Shape
5. Drag and drop Polyshape object to the Project window to turn it into a Prefab

Expected Result: No errors are thrown and a Polyshape Prefab is created
Actual Result: ArgumentNullException is thrown when Polyshape Prefab is created

Reproducible with: 2021.1.0a1(4.4.0), 2021.1.0a8(5.0.0-pre.7)
Not reproducible with: 2018.4.31f1(4.4.0), 2019.4.20f1(4.4.0, 5.0.0-pre.7), 2020.2.3f1(4.4.0, 5.0.0-pre.7)

Notes:
1. Occurring on Windows 10 & Mac 10.15.

Stack trace:
ArgumentNullException: Value cannot be null.
Parameter name: key
System.Collections.Generic.Dictionary`2[TKey,TValue].FindEntry (TKey key) (at <9577ac7a62ef43179789031239ba8798>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].TryGetValue (TKey key, TValue& value) (at <9577ac7a62ef43179789031239ba8798>:0)
UnityEditor.EditorTools.EditorToolUtility.GetIcon (System.Type editorToolType) (at <5bcc5242f4ee487aa687ca1377112811>:0)
UnityEditor.Toolbars.LastCustomToolButton.UpdateContent () (at <b1e0fe3c31bd4291b482ea3bf8ed412c>:0)
UnityEditor.Toolbars.LastCustomToolButton.OnSelectionChanged () (at <b1e0fe3c31bd4291b482ea3bf8ed412c>:0)
UnityEditor.Selection.Internal_CallSelectionChanged () (at <5bcc5242f4ee487aa687ca1377112811>:0)