You are getting your stockItem from the $observer however you've changed the parameters of your function, therefor $stockItem = $observer->getEvent(); can't get anything because $observer does not exist...

You should have :

private function lowStockReportSendEmail($observer)
{
    $stockItem = $observer->getEvent();
[...]

You are getting your stockItem from the $observer however you've changed the parameters of your function, therefor $stockItem = $observer->getEvent(); can't get anything because $observer does not exist...

From your OP we can see you are using the $observer inside the lowStockReportSendEmail function, however the observer is not present here!

Assuming you have the observer, and in that function you create $product and $toEmailAddress before calling lowStockReportSendEmail you should have :

private function lowStockReportSendEmail($observer, $product, $toEmailAddress)
{
    $stockItem = $observer->getEvent();
[...]

And in the code that calls the funtion, obviously $this->lowStockReportSendEmail($observer, $product, $toEmailAddress);