[DQT] Define field tab - cannot select a Visit in Visits filter

[laemtl]

In DQT > Define field tab, the checkboxes in the Visits menu dropdown are not selectable when no fields are selected.

If this is the intended behavior, I suggest adding a visual indication that those checkboxes are disabled.

With some console debugging, I noticed the click event is fired twice.
Observed in Chrome 75.0.3770.80 and Firefox 76.0.1.

To reproduce

• Go to Reports > Data Query Tool
• Select the Define field tab
• Select an instrument
• Interact with the Visits filter on the right