Regular Panel will not close

I have an Objective-C app running on MacOS using XCode 14.1, in which there are 3 levels of form. The Level 2 form is opened from an action with a makeKeyAndFront action from a button. The Level 3 form is a regular panel opened from the level 2 form in a similar way. The level 3 form can be closed normally if no action changes the level 2 form. However, if an image is loaded into an NSImageView control on the level 2 form, the level 3 form can be displayed but cannot be closed by clicking on the red x button which normally performs this function. The level 3 form can be minimised. What do I have to do to allow the level 3 form to be closed in this situation?