Need to convert a network buffer polygon (that has gaps) into a solid polygon that extends to the maximum edge?

[ErikaMartino]

I need to convert a network buffer polygon (that has gaps) into a solid polygon that extends to the maximum edge?

For example, from this...

to this:

[DanPatterson_Retired]

duplicate of this thread https://community.esri.com/thread/182142-i-need-to-convert-a-network-buffer-polygon-that-has-gaps-in...