Popup class declares member name _current but use current instead

[peishaofeng]

Wekan version:
from 1.53.0 to latest

Problem description:

• in client/lib/popup.js, _current is declared as class member but current is actually used.

_current is declared in constructor

    // We only want to display one popup at a time and we keep the view object
    // in this `Popup._current` variable. If there is no popup currently opened
    // the value is `null`.
    this._current = null;

but use current after render embedded template

      if (!self.isOpen()) {
        self.current = Blaze.renderWithData(self.template, () => {
          self._dep.depend();
          return { ...self._getTopStack(), stack: self._stack };
        }, document.body);