problem causes:

After mouted loads the table data, the hidden pop-up box is not compiled and rendered into the dom. When the click pops up, $refs does not get the dom element, resulting in’resetFields’ of undefined.

solution:

If it is the first time you click to add, there is no need to reset, according to the element undefined judgment

resetForm() {
       if ( this .$refs.userEditRef !== undefined) {
         this .$refs.userEditRef.resetFields()
      }
}