Modal dialog opened from view model gets separated from main window on switching to other application

I have a Modal dialog in my application. I am opening the dialog from a command in view model using window.ShowDialog() .

The problem is that when we switch to other opened application and then clicking on our app in taskbar hides the modal dialog. I have figured out that the problem is due to Window ownership.

i can't assign the owner to modal dialog from view model. Please help.