System Information
Operating system: Windows 10 x64 (20H2)
Graphics card: Nvidia RTX 2070S

Blender Version
Broken: 2.91.0
Worked: ?

Short description of error
If a driver is added on a numeric value, the picker widget (e.g. to pick the object of the traget property) cannot be used. Instead of a normal cursor (arrow), the cursor for value selection (horizontal double-arrow) remains.
When trying to pick an object, the popup-driver editor closes immediately. The same applies when the driver is edited, using the context menu.

Edit: The picker seems not to work at all in the popup window, regardless of the value type of the applied field

Picking only works in the "Drivers" Editor (Shift F6)

See attached animation

Exact steps for others to reproduce the error

1. Select an object
2. Hover over a numeric field (e.g. Location X)
3. Right-Click
4. Select "Add Driver" in the context menu
   • The popup window to edit the driver opens
5. Click on "Add Input Variable"
6. Try to use the object picker on "Prop:"
   • The picker cannot be used