Arena: crash when logging under-construction StructureTower object

[wtfrank]

The following console.log line crashes when the under-construction structure member is StructureTower:

      let cs = getObjectsByPrototype(ConstructionSite);
      for (const c of cs) {
        if (c.my) {
          console.log("my cs:", c.structure, c.structure == null, c.structure === undefined, c.progress, c.progressTotal);