Merge pull request #555 from argonui/import-instructions

Added nil handling for import instructions

src/arkhamdb/ArkhamDb.ttslua

@@ -323,6 +323,7 @@ do
         if instructor == "add:" then
           slots[str] = (slots[str] or 0) + 1
         elseif instructor == "remove:" then
+          if slots[str] == nil then break end
           slots[str] = math.max(slots[str] - 1, 0)
         end
       end