Fix modal bug

Created by: ghost

Fixes #24117 (closed).

Note that the hide() function checks, before executing the routine, that the modal is not hidden. This is logical: why should I try to hide something that is already hidden?

hide(event) {
  if (this._isTransitioning || !this._isShown) {
    return
  }
}

However, the show() function does not do this same check and runs the entire routine even though the modal is visible.

show(event) {
  if (this._isTransitioning) {
    return
  }
}

That is why I check, before running the whole routine, that the modal is not visible. And I have verified that this indeed solves the bug.

PS: Sorry for the English. I'm using Google Translator.