To correct the image with stitching gap in the top, you can use the zenith optimization function during preview in Stitcher.

To explain in details, this is a new function added to Stitcher.

When previewing the outcome stitching effect, you can turn on the zenith optimization function to optimize the stitching effect in the top. This is especially useful for images with obvious stitching gap in the top.

Let's take a look at the effect:

Before optimization:

After optimization: