This information applies to versions 7 and older
This is normal behavior: while working with a Plug-in, the host software is not responsive. You might even see this reflected in the Windows Task Manager / Activity Monitor (Mac). As soon as you close the Plug-in, your host software will return to its usual state.
Nik Collection 8 introduced a warning message that remains active as long as the plug-in is open to inform you about this.