Sorry for the late update.
Space Guy modified the editor to look elsewhere for heightmaps. Line 1985 of EditorGui.cs says
$TerraformerHeightfieldDir = "editor/heightScripts"; //badspot: removed common/
This worked a couple updates ago, but as I recall the mission editor has been removed from the Blockland website and neither editor nor common are default directories anymore so we can't use them.
If you open it up and change that line to
$TerraformerHeightfieldDir = "add-ons/heightscripts";
then create a folder in your add-ons folder called heightscripts it will work. I don't really like throwing random folders in add-ons like that but I think putting it in base or something is an even worse idea. If you really want to keep your add-ons folder neat then you may be able to set it to ./ and just add all your heightmaps to the zip file. If you're making maps though your add-ons folder is probably already a giant mess with lots of unfinished maps so a stray heightmaps folder shouldn't be too big of a deal.
And here's just an optional but cool thing you can do: open up editorRender.cs and look at line 28:
%editor.consoleFillColor = "0 0 0 0";
change it to
%editor.consoleFillColor = "255 128 0 64";
It will make your spawn spheres fill in a transparent orange color so you can see the intersection with interiors and terrain better.