The ExpressionViewer does not set its 'expression' attribute when selecting a value from the auto-complete pop-up using mouse selection. Note that typing the value directly and selecting but using arrow keys + enter key correctly sets the value.
Steps to replicate:
1. Open viewer
2. Start typing until auto-complete pop-up shows
3. Select value using mouse selection (i think this is a double-click)
4. Check expression by: viewer.getExpression()