How to reproduce:
1. Create new project
2. Add styles.uss file and E03_StyleSheet.cs file to the project
3. Open UIElementsExamples > 03_StyleSheet window
4. In a text editor, make a change in both the styles.uss file and E03_StyleSheet.cs file (like add a space)
5. Focus Unity again

Reproducible with: 2019.1.0a12(error appear), 2019.1.8f1(error only), 2019.2.0a13(blank window appears), 2019.2.0b8, 2019.3.0a8
Not reproducible with: 2017.4.29f1, 2018.4.3f1(UI elements not available), 2019.1.0a11

Expected: Nothing should break
Actual results: You'll get some null ref error in the console and the 03_StyleSheet window will go blank. You'll have to close it and reopen it to fix it