优化:避免错误

src/Add-on/ACAD/DxfEntityConvert.ts

@@ -23,7 +23,7 @@ export function Dxf2Entitys(dxfstr: string)
     let parser = new DxfParser();
     let dxfDocument = parser.parseSync(dxfstr);
     let ents: Entity[] = [];
-    for (let en of dxfDocument.entities)
+    for (let en of dxfDocument?.entities)
         Conver2WebCADEntity(en, dxfDocument, ents);
     return ents;
 }