@Progi1984 Due to the change we get an error with SlideLayouts:

PhpOffice\PhpPresentation\Slide\AbstractSlide::getRelsIndex(): Return value must be of type string, null returned.

It seems that relsIndex is not set for SlideLayouts and remains null. By adjusting the return type it does not work now. The method is used in:

PhpOffice\PhpPresentation\Writer\PowerPoint2007\PptSlideLayouts::67

I guess it can be fixed by setting relsIndex in

PhpOffice\PhpPresentation\Writer\PowerPoint2007\ContentTypes::75

like

$oSlideLayout->setRelsIndex((string) $oSlideLayout->layoutNr = ++$sldLayoutNr);

@Progi1984 I have created a PR for it: #646

@Progi1984 Could you please merge this PR?