UI Toolkit debugger throws exceptions when inspector Package manager window
Reported by Ethan Li

Steps to reproduce:
1) Open any Unity project
2) Open Package Manager window
3) Inspect any element under the root visual element

Expected: no exception is thrown and selectors are displayed
Result: exception occurs

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.UIElements.Debugger.MatchedRulesExtractor.<FindStyleSheets>g__RecursivePrintStyleSheetNames|4_0 (UnityEngine.UIElements.StyleSheet importedSheet, UnityEditor.UIElements.Debugger.MatchedRulesExtractor+<>c__DisplayClass4_0& , UnityEditor.UIElements.Debugger.MatchedRulesExtractor+<>c__DisplayClass4_1& ) (at /Users/antoine/Documents/unity-imgui/External/MirroredPackageSources/com.unity.ui/Editor/Debugger/StylesDebugger.cs:399)