This happens because you have a malformed addon trying to install. 1) Go to Tools-> Addons, select the Installation panel, if it exists, and cancel any ongoing addon installations. Restart. Problem should be solved. OR 2) If the Installation panel does not exist, disable every extension and restart. The problem should be gone. If you want any of the addons, enable one-by-one and restart until the problem comes back. As soon as it comes back, proceed to disable the last extension you enabled and to enable all the other extensions. One of these should work.