Maybe I am missing something or I am doing something terrible wrong - so sorry if it is true

I want to show "Viewer" in new thread. I did sample aplication. When I am running my code I got some strange exceptions (that something is my new thread is missing).
Why I want viewer in new thread? When I run "Viewer" in the main thread, the application stops being responsive (I can not click anything with the mouse). I want to despite of running "Viewer", my application to be still responsive. I thought to run "Viewer" in a new thread, but apparently encountered some difficulties.
I would add that I want to be able to run even a few "Viewer", so that the application was still responsive.
Thank you in advance for your help.