No. It is never safe. This is a carbon monoxide poisoning risk. A cracked door does not provide sufficient ventilation.
Operate the generator OUTSIDE only, far away from windows, doors, and vents. The exhaust must be directed away from occupied structures. Keep it at least 3 feet (1 meter) from buildings.
Do not operate in a house, garage, basement, crawlspace, trailer, or boat—even if partly enclosed. The CO-MINDER safety system may shut it off, but it cannot prevent all danger. Exhaust contains toxic carbon monoxide that can kill you in minutes.
HEADS UP: For wet conditions, keep the generator dry. Do not use in rain or snow. If stored outdoors, check all electrical components on the control panel for moisture or ice before each use to prevent shock hazard.