Error if ticket is opened as a guest, then replied to by a logged in client

[xyzulu]

Just in case this happens to anyone else. WHMCS have a hotfix that resolves this issue. It seems you'll need to contact support to be provided it however. The issue is CORE-15504

Both admin and customer will receive an error and be unable to access the ticket. The error is:

Error: Call to a member function owner() on null in .../vendor/whmcs/whmcs-foundation/lib/Support/Traits/Requestor.php:0
Stack trace:
#0 .../vendor/whmcs/whmcs-foundation/lib/Support/Traits/Requestor.php(0): WHMCS\Support\Ticket\Reply->getRequestorType()
#1 .../vendor/illuminate/database/Eloquent/Concerns/HasAttributes.php(428): WHMCS\Support\Ticket\Reply->requestor()
#2 .../vendor/illuminate/database/Eloquent/Concerns/HasAttributes.php(414): Illuminate\Database\Eloquent\Model->getRelationshipFromMethod('requestor')
#3 .../vendor/illuminate/database/Eloquent/Concerns/HasAttributes.php(370): Illuminate\Database\Eloquent\Model->getRelationValue('requestor')
#4 .../vendor/whmcs/whmcs-foundation/lib/Model/AbstractModel.php(0): Illuminate\Database\Eloquent\Model->getAttribute('requestor')
#5 .../vendor/illuminate/database/Eloquent/Model.php(1633): WHMCS\Model\AbstractModel->getAttribute('requestor')
#6 .../admin/supporttickets.php(0): Illuminate\Database\Eloquent\Model->__get('requestor')
#7 {main}

The hotfix resolves the issue and the ticket is once again viewable.