Stacktrace when clicking "Browse" button next to "Content path" in HST Configuration

Stacktrace when clicking "Browse" button in HST Configuration. After this, you need to log out/log in or refresh the page to be able to do anything else.

To reproduce (on Go Green test server):

• Go to Channel Manager
• Click for example "Dutch website"
• Click Edit
• Click the Settings icon and "Edit HST Configuration"
• Click any folder in the list
• Click the "Browse" button next to the "Content path" input field, which looks like: [...]

Expected behaviour: A dialog should open
Actual behaviour: Nothing happens. After this, nothing happens when clicking anything else but "Logout"

Attached log file with stacktrace.

Found during regression tests