Installation issue

[anytizer]

Fatal error: Uncaught TypeError: imagedestroy(): Argument #1 ($image) must be of type GdImage, null given in D:\xampp\htdocs\cpg1.6.x-1.6.12\install.php:2034 Stack trace: #0 D:\xampp\htdocs\cpg1.6.x-1.6.12\install.php(2034): imagedestroy(NULL) #1 D:\xampp\htdocs\cpg1.6.x-1.6.12\install.php(1368): GDtest->testTextOnImage() #2 D:\xampp\htdocs\cpg1.6.x-1.6.12\install.php(348): testImageProcessor() #3 {main} thrown in D:\xampp\htdocs\cpg1.6.x-1.6.12\install.php on line 2034

Variable $source_a seems not to be defined previously at https://github.com/coppermine-gallery/cpg1.6.x/blob/develop/install.php#L2034