Undesired filling in of empty spaces

[dylanhoen]

I made my first 3D object in MatterHackers MatterControl2.0.
It is half a skeleton of a cube made out of joined, overlapping, right-triangle prisms, with holes drilled through the 4 bottom corners. The middle is all empty space.
I exported it to .STL, and opened it in Simplify3D.
It looks all right in initial view:

, but in prepare to print, it fills the bottom middle up with stuff:

I have Skirt/Brim checked, but the other check boxes on the Additions tab are unchecked.
What is going on?

Dylan

[dylanhoen]

I forgot about this post. It seems the .stl file caused similar problems in several slicers. It probably had leaks in its surface. I stopped using MatterControl and started using the TinkerCad webpage instead.