How do I revert to a previous version of TC?

I am having the same issues. 

Unable to replay or create new tests. 

Also tried to run sample project from TC. Same result. 

Version 15.50 did not work for me. Still trying to figure out how to find downloads previous versions. License manager shows only one version 😞

Submitted tech support ticket. Still waiting. 

The window with the handle 0x00000000 does not exist.

This error occurred when the tested window (or control) or its parent object was deleted or recreated right before the method call. For example, you stored a window reference to a variable and then called a method through this variable.
If the object has been recreated, then to avoid the error, get the new object before the method call.

Tested object:
Aliases.browser.....