Navigation stack error while launching

edit

In my costmap_common_param.yaml file i've placed these two lines :

observation_sources: laser_scan_sensor
laser_scan_sensor: {sensor_frame:base_scan, data_type: LaserScan, topic: scan, marking: true, clearing: true}

When i am launching my nav_pkg i get this error :

load_parameters: unable to set parameters (last param was [/move_base/TrajectoryPlannerROS/holonomic_robot=False]): cannot marshal None unless allow_none is enabled

I've been searching for a while and i found a comment which says that this error means one of the parameters is null (undefined or an empty string). It also says to add the following line when roslaunching the nav_pkg :

--dump-params

Indeed when i did it i found this :

/move_base/global_costmap/laser_scan_sensor/sensor_frame:base_scan: null

But i have no ideas why is this happening and also i don't know how to fix it. Any suggestions ?