Sub editor doesn't allow creating structures with zero width/height, fixed null exceptions when trying to remove a structure without wall sections

Subsurface/Source/Map/Structure.cs

@@ -469,14 +469,18 @@ namespace Barotrauma
                     GameMain.World.RemoveBody(b);
             }
 
-            foreach (WallSection s in sections)
+            if (sections != null)
             {
-                if (s.gap != null)
+                foreach (WallSection s in sections)
                 {
-                    s.gap.Remove();
-                    s.gap = null;
+                    if (s.gap != null)
+                    {
+                        s.gap.Remove();
+                        s.gap = null;
+                    }
                 }
             }
+
             if (convexHulls != null) convexHulls.ForEach(x => x.Remove());
         }