Problem:

When editing an existing string variable dashboard filter that has "Null/undefined" as allowed value, it appears twice in the list of all values

Steps to reproduce:

• create a new dashboard
• add a report with a string variable to the dashboard
• add a variable filter for the string variable to the dashboard and check the "Null/undefined" value
• edit the variable filter

Actual result :

• Null/undefined appears twice in the list of values

Expected result:

• Null/undefined appears only once

Note: this behaviour also exists in report filters.