fix(Tinymce): When content is null,set content to '' to avoid error

src/components/Tinymce/index.vue

@@ -50,6 +50,7 @@ export default {
   watch: {
     value(val) {
       if (!this.hasChange && this.hasInit) {
+        if(val === null) val = ''
         this.$nextTick(() => window.tinymce.get(this.tinymceId).setContent(val))
       }
     }