Multiple errors are thrown on creating "New USS Selector" with name Single character.
-Failed to calculate selector specificity error
-NullReferenceException error.
-Assertion failed error.

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. StyleSheets > Click on "+" > Create New USS > File Name > Testuss > Save.
4. StyleSheets > Select Testuss > Inspector > Selector > Type "w".
5. Click on "Create New USS Selector".

Actual Result:
Unable to create "New USS Selector" in Stylesheet.
Multiple errors are thrown in the console.

Expected Result:
USS Selector should be created without any error in the console.

Reproducible in:
Multiple errors are thrown on creating "New USS Selector" with name Single character.
-Failed to calculate selector specificity error
-NullReferenceException error.
-Assertion failed error.

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. StyleSheets > Click on "+" > Create New USS > File Name > Testuss > Save.
4. StyleSheets > Select Testuss > Inspector > Selector > Type "w".
5. Click on "Create New USS Selector".

Actual Result:
Unable to create "New USS Selector" in Stylesheet.
Multiple errors are thrown in the console.

Expected Result:
USS Selector should be created without any error in the console.

Reproducible in:
2021.1.0a10, 2020.2.0f1, 2020.1.0f1, 2019.4.0f1 with Package version 1.0.0-preview.9.

Note: Please Find attached Stacktrace.

Environment:
Occurring on Windows 10 & Mac 10.15.

Note: Please Find attached Stacktrace.

Environment:
Occurring on Windows 10 & Mac 10.15.