The button is covered by the unknown grey bar which is not in the hierarchy of views [on hold]

(Upload the link of the image) I am working on the project. I use storyboard to organise the view. It happened that when I put the button at the bottom of the parent view, there is a grey bar beyond it. However, I cannot find out this grey bar on the view's hierarchy. Also, this issue did not have any impact on the App at run time.

Please see the following image. Image Link

Does anyone has the idea about this issue?