SSMS 2016 Query Store: Missing Index Details error: DocumentFrame (SQLEditors)
It is happening to me from Query Store query plan screens on SSMS 13.0.16106.4 as well.

Workaround that I found, until they fix it: 


Press "New Query" button for a blank new tab
Switch back to the Query Store tab and right-click, select "Missing Index Details" again
It'll open up the missing index details in a third tab


When you look at another, you may have to repeat the steps, even if you still have the blank tab open.