docs: def _check_file(self):
          """
          Checks watched file moficiation time and permission changes.
          """
          try:
              self.editor.toPlainText()
          except RuntimeError:
              self._timer.stop()
              return
          if self.editor and self.editor.file.path:
              if not os.path.exists(self.editor.file.path) and self._mtime:
                  self._notify_deleted_file()
              else:
                  mtime = os.path.getmtime(self.editor.file.path)
                  if mtime > self._mtime:
                      self._mtime = mtime
                      self._notify_change()
                  # check for permission change
                  writeable = os.access(self.editor.file.path, os.W_OK)
                  self.editor.setReadOnly(not writeable)