Value src dropdown does not have active state

[kbultman]

Describe the bug
When selecting a value source during a rule, the dropdown does not have an active state and is therefore invisible unless you hover and see the caret

To Reproduce
Go to demo: https://ukrbublik.github.io/react-awesome-query-builder/
Hover over login value and see dropdown to left

Expected behavior
Value source dropdown should have an active state or placeholder on dropdown for visibility

Screenshots