修改

src/components/main.vue

@@ -735,7 +735,10 @@ const rules = {
   targetSubjectId: [
     {
       validator(_, value) {
-        return value && value !== subjectData.value.sourceSubjectId
+        if (value === null || value === undefined) {
+          return true
+        }
+        return value !== subjectData.value.sourceSubjectId
       },
       trigger: ['blur', 'change'],
       message: '目标要素不能与源要素相同',
@@ -744,7 +747,10 @@ const rules = {
   sourceSubjectId: [
     {
       validator(_, value) {
-        return value && value !== subjectData.value.targetSubjectId
+        if (value === null || value === undefined) {
+          return true
+        }
+        return value !== subjectData.value.targetSubjectId
       },
       trigger: ['blur', 'change'],
       message: '源要素不能与目标要素相同',