How to reproduce:
1. Create a new project or open an existing one
2. Open the Bug Reporter (Help -> Report a Bug...)
3. Click the Add File button just after the Bug reporter opens
4. Try to select a file/folder in the file selection window

Expected result: File/folder is selected
Actual result: File/folder is not selected

Reproducible with: 2017.4.31f1, 2018.4.5f1, 2019.1.13f1, 2019.2.0f1, 2019.3.0a5

Notes:
The issue seems to only reproduce if the Add File button is clicked just after the Bug Reporter opens. Waiting for a second before clicking the button will fix the issue.
When the issue occurs, the file selection window will only start to work again after restarting the Bug Reporter.
The issue does not reproduce on Unity version 2019.3.0a6 and up.