Our Regedit can be launched multiple times, unlike MS one

To reproduce the bug, simply try to launch Regedit multiple times by double clicking on its executable file (regedit.exe) or link to it.
It is possible to launch multiple instances of our Regedit, which is not allowed in MS one. Attached GIFs show the described comparison as well.

I made a fix for this and will submit a PR soon. After my fix, opening Regedit several times does no longer launch many instances of it, but there is still some difference with MS behaviour: when trying to launch the app second time, the window of current launched instance doesn't became active (to point the user that it is already launched), but in Windows Regedit it does. So it also needs to be fixed. But I don't know exactly how to do it. I need some help.