editor: fix crash on sector creation

source/build/src/build.cpp

@@ -7076,11 +7076,12 @@ end_join_sectors:
                     yax_update(0);
                     yax_updategrays(pos.z);
 #endif
-                    calc_sector_reachability();
                     numwalls = newnumwalls;
                     newnumwalls = -1;
                     numsectors++;
 
+                    calc_sector_reachability();
+
                     asksave = 1;
                     printmessage16("Inner loop made into new sector.");
                 }