MovieClip object becomes null on its own.

No, the app is a mobile app, so different views (frames) have different set of items. So in one frame I display game stas score, name, etc. while in another one I have a FAQ sections. The object in frame 3 doesn't exist in frame 10. This is undoubtedly a very annoying bug. I fixed it by placing the object on a separate layer and extending through the entire timeline, so that it's available in every view. I just added a new keyframe (instance name is preserved all the time), so that I could move the object out of users signt (it's an absolutely terrible workaround). I didn't want to play with the "visible" property or adding that object through code because I have several navigation buttons and I would have to add too many add/remove listeners and add/remove child, or the visible property.

Flash/animate cc can randomly lose instance names of random objects when navigating through frames.It's already happened to me 3 times on unique objects (used only once) or reused objects, e.g. same back button, but with different instance names. It's completely random.