What version are you running?
Reviewboard release-4.0.x

What's the URL of the page containing the problem?
/admin/

What steps will reproduce the problem?

On release-4.0.x, navigate to /admin/ and load the dashboard normally.
Add a new widget onto the page, using extensions or "Add Small Widgets" button.
Reload the /admin/ page.


What is the expected output? What do you see instead?
The expected output is that the admin dashboard should reload, with your new widget displaying on the page. Instead, the page crashes with a "Key Error". 

What operating system are you using? What browser?
I'm on Ubuntu 18.04, using Firefox 62.0 (64-bit)

Please provide any additional information below.
We suspect this is because the new widget is not given a position in the page, if the dashboard was already generated for the user. 
We broke this during development, when attempting to add a new secondary widget into the dashboard. If you had release-4.0.x /admin/ loaded on a page, then added code that added a new widget, then reloaded the page, that would also trigger the page to have a key error.