Bug report: GetLicense with TextMask = null fails

According to the docs:
https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_PlugIns_PlugIn_GetLicense_1.htm
passing TextMask = null like this should work:

GetLicense(LicenseCapabilities.CanBeSpecified, null, MyValidateProductKey, MyOnLeaseChangedDelegate);

but it consistenly fails with:

2021-02-17 17:34:11.578 Rhinoceros[4088:573565] Xamarin.Mac: Aborting due to trying to marshal managed exception:
Object reference not set to an instance of an object (System.NullReferenceException)
  at ValidationClient.UI.Views.EnterLicenseKeyView.licenseKeyTextChanging (System.Object sender, Eto.Forms.TextChangingEventArgs e) [0x0000b] in /Users/bozo/TeamCity/buildAgent/work/96e64af5b81c6f85/src4/Zoo/ValidationClient/Views/EnterLicenseKeyView.cs:61 
  at Eto.PropertyStore.TriggerEvent[T] (System.Object key, System.Object sender, T args) [0x0000e] in /Users/bozo/TeamCity/buildAgent/work/96e64af5b81c6f85/src4/DotNetSDK/Eto/src/Eto/PropertyStore.cs:238 
  at Eto.Forms.TextBox.OnTextChanging (Eto.Forms.TextChangingEventArgs e) [0x00000] in /Users/bozo/TeamCity/buildAgent/work/96e64af5b81c6f85/src4/DotNetSDK/Eto/src/Eto/Forms/Controls/TextBox.cs:73 
  at Eto.Forms.MaskedTextBox.OnTextChanging (Eto.Forms.TextChangingEventArgs e) [0x00000] in /Users/bozo/TeamCity/buildAgent/work/96e64af5b81c6f85/src4/DotNetSDK/Eto/src/Eto/Forms/Controls/MaskedTextBox.cs:454 
  at Eto.Forms.TextBox+Callback.OnTextChanging (Eto.Forms.TextBox widget, Eto.Forms.TextChangingEventArgs e) [0x00038] in /Users/bozo/TeamCity/buildAgent/work/96e64af5b81c6f85/src4/DotNetSDK/Eto/src/Eto/Forms/Controls/TextBox.cs:347 
  at Eto.Mac.Forms.MacFieldEditor.ShouldChangeText (Foundation.NSRange affectedCharRange, System.String replacementString) [0x0001d] in /Users/bozo/TeamCity/buildAgent/work/96e64af5b81c6f85/src4/DotNetSDK/Eto/src/Eto.Mac/Forms/MacFieldEditor.cs:143 

Passing in a TextMask like this works:

GetLicense(LicenseCapabilities.CanBeSpecified, ">S\\AMP\\LE-AAAAAAAAAA", MyValidateProductKey, MyOnLeaseChangedDelegate);