module - Magento get invoice number -

i'm trying build module magento gets invoice number. main problem fatal error: call member function getincrementid() on non-object...on line 19 ($invoice creation line). function is:

 $invoice = $observer->getevent()->getinvoice()->getincrementid();     $last4 = substr($invoice, -4);      $shipment = $observer->getevent()->getshipment();     $track = mage::getmodel('sales/order_shipment_track')             ->setnumber($last4) //tracking number / awb number             ->setcarriercode('custom') //carrier code             ->settitle('custom'); //carrier title     $shipment->addtrack($track); 

try replacing first line of code this

$invoice = $observer->getevent()->getorder()->getincrementid(); 

using getinvoice() method order may returning null give error have on line 19.