Is it safe to call disconnect with a deleted object as sender or receiver argument?

Question

I have an application with a QMdiArea. There's some incoming and outgoing signals connected to the widget in the currently active subwindow. Whenever the active subwindow changes I want to disconnect all the connections to the previously active widget and connect to the newly activated widget. As such:

//Function connected to QMdiArea::subWindowActivated...
void
MainWindow::SubWindowActivated(QMdiSubWindow* subWindow)
{
    auto activeWidget{ qobject_cast<MyWidget*>(subWindow->widget()) };
    if (activeWidget == mPreviouslyActiveWidget)
    {
        return;
    }

    //disconnect all incoming and outgoing signals between previously active widget and this.
    disconnect(this, nullptr, mPreviouslyActiveWidget, nullptr); 
    disconnect(mPreviouslyActiveWidget, nullptr, this, nullptr);

    //re-establish connections to activeWidget ... removed for brevity
    mPreviouslyActiveWidget = activeWidget;
}

It's possible that the subwindow changed because the previous subwindow was closed by the user and thus no longer exists/is deleted by the QMdiArea. In that case I would be calling the disconnect functions whith mPreviouslyActiveWidget pointing to a deleted object. Is this a problem? Will the call simply fail and return false or is this undefined ?