You really did not fix it if you used "==" in
{w.label == "_";}
the above statement is actually not an assignment to the label but instead just evaluating the condition which reports true if the label of the widget matches "-"

Hoping this is just copy/paste or retype error on your part.

A better approach along to set the label to "-" if timer is not numeric is:

if (isNaN(hallLightsTimer)) {w.label ="-";}

[Link: w3schools.com]