I found a bug in NIS4 with windows closing.
I repeated this case 3 times and the same occurred.
i start full system scan, at the end Norton Antivirus found 4 trojans and display dialog with options to repair or delete them.
I click delete, each one step by step, than all buttons become greyed and the window red dot for exit too.There is no way to close Antivirus window.The only way is force quit.