Describe the bug
Just tried the "Demo code" to merge 2 files and it doesn't work.

To Reproduce
My code is simple :

(some code before to set variable with file path)
$ppt_transition = $this->ppt_transition;
$basePPTX = new PPTX($ppt_transition);

$ppt_introduction = $this->ppt_introduction;
$endPPTX = new PPTX($ppt_introduction);

$basePPTX->addSlides($endPPTX->getSlides());

Expected behavior
Not throwing erros ...

Screenshots
Error is the following

Fatal error: Uncaught TypeError: Return value of Cristal\Presentation\Resource\XmlResource::getUniqueID() must be of the type integer, float returned in F:\Developpement\Page Oueb\abondonworship\classes\utils\ppt\Cristal\Presentation\Resource\XmlResource.php:229 Stack trace: #0 F:\Developpement\Page Oueb\abondonworship\classes\utils\ppt\Cristal\Presentation\Resource\Presentation.php(39): Cristal\Presentation\Resource\XmlResource::getUniqueID() #1 F:\Developpement\Page Oueb\abondonworship\classes\utils\ppt\Cristal\Presentation\PPTX.php(173): Cristal\Presentation\Resource\Presentation->addResource(Object(Cristal\Presentation\Resource\SlideMaster)) #2 F:\Developpement\Page Oueb\abondonworship\classes\utils\ppt\Cristal\Presentation\PPTX.php(125): Cristal\Presentation\PPTX->addResource(Object(Cristal\Presentation\Resource\Slide)) #3 F:\Developpement\Page Oueb\abondonworship\classes\utils\ppt\Cristal\Presentation\PPTX.php(212): Cristal\Presentation\PPTX->addSlide(Object(Cristal\Presentation\Resource\Slide)) #4 F:\Developpement\ in F:\Developpement\Page Oueb\abondonworship\classes\utils\ppt\Cristal\Presentation\Resource\XmlResource.php on line 229

Desktop (please complete the following information):

• OS: [Windows]
• PHP Version: [ PHP 7.1.3 x86]
• Software used to open the final PPTX [Powerpoint]

Additional context
I can open "template" and save both files, but merging them trows an error that looks like a bug from your project