Fcu_url: Address already in use

I am trying to launch a gazebo environment with one erle-copter.

I am launching mavros as follows:

<    include file="$(find mavros)/launch/apm_sitl.launch" >
    		<arg name="fcu_url" value="udp://" /> 
    		<arg name="gcs_url" value="" /> 
    		<arg name="tgt_system" value="1" />
    		<arg name="tgt_component" value="1" /> 

I have the following fatal error:

[FATAL] [1530614431.591097132, 26.102500000]: FCU: DeviceError:udp:bind: Address already in use
================================================================================REQUIRED process [uav/mavros-1] has died!
process has finished cleanly
log file: /home/avgi/.ros/log/5bf6e9ee-7ead-11e8-8303-844bf591bb91/uav-mavros-1*.log
Initiating shutdown!

thank for helping me!!!