Trouble exporting map from RTAB-Map

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I am on ROS Kinetic, but rtabmap is the same version than on Melodic and I cannot reproduce the problem. Following exactly what you did, after doing rosservice call /publish_map 1 1 0, I see in the rtabmap_ros terminal:

[ INFO] [1551304861.803715934]: rtabmap: Publishing map...
[ WARN] [1551304862.123340986]: Many occupancy grids should be loaded (~698), this may take a while to update the map(s)...
[ WARN] [1551304872.284061124]: Map(s) updated! (10.160718 s)
[ WARN] [1551304872.284428237]: /proj_map topic is deprecated! Subscribe to /grid_map topic instead.

so the map is created, then on map_server terminal, I can see:

$ rosrun map_server map_saver map:=proj_map
[ INFO] [1551304856.764290592]: Waiting for the map
[ INFO] [1551304872.285442346]: Received a 551 X 447 map @ 0.050 m/pix
[ INFO] [1551304872.285460183]: Writing map occupancy data to map.pgm
[ INFO] [1551304872.289321822]: Writing map occupancy data to map.yaml
[ INFO] [1551304872.289390615]: Done

The map created:

The warnings that it did not receive any data since 5 seconds are normal, as no sensors are publishing topics to rtabmap.