I got help in another forum and found the solution to that problem:
QMdiSubWindow *previous = ui->mdiArea->activeSubWindow();
QTextEdit *edit=new QTextEdit;
QMdiSubWindow *sub=ui->mdiArea->addSubWindow(edit,Qt::Window)
sub->show();
if (previous==0)
ui->mdiArea->setActiveSubWindow(sub);
else if (previous->isWidgetType()) //I check if previous is widget, because if you close the previous tab, when the new one is opened the program crashes on the next line because previous no longer exists.
ui->mdiArea->setActiveSubWindow(previous);