Notice: Trying to get property of a non-object in /site/modules/PagePermissions.module on line 663 on single template

[hellomoto]

Line 663 =             if(count($page->template->childTemplates) && !in_array($addPage->template->id, $page->template->childTemplates)) {

The template has no child templates and is the sole child template of its parent template; why this notice?

[kixe]

Where and how did you define $addPage ? Try

var_dump(gettype($addPage)); // 'object'
var_dump(get_class($addPage)); // 'ProcessWire\Page'