A client installed and uninstalled some monitoring software on their 365 Tenancy and this has left behind the artefact that all the sites active at the time contain:
var g_wsaRUMEnabled = true;
This in turn means that some pages on load are throwing an error:
Uncaught TypeError: Cannot read property 'length' of null at SP.RUMLogger.$U_0 (sp.core.js:2) at SP.RUMLogger.SetPageTransitionType (sp.core.js:2)
In particular this error happens on load for the "Manage App Deployments" page in the app catalog which breaks the page entirely.
I've never personally come across this RUM Enabled setting - I assume it's something to do with Real User Monitoring but frankly I don't care. All I really want to be able to do is turn it off so I can actually use the notoriously unscriptable "Manage App Deployments" page.
[apologies if multi posted - the site is throwing an error on 'Post']