load custom world in gazebo from a launch file

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Hello everyone,

I've built a custom wold for gazebo (dae file) and I'm trying to load this file from a launch file. When I run the world file directly from gazebo, it woks and I can see the world (gazebo maze.world). But when I build a launch file and include the world file inside the launch file, I get the following error:

Error [SystemPaths.cc:367] File or path does not exist[""]
Error [Visual.cc:2138] No mesh specified

Here is the content of the world file (maze.world):

and here is the launch file(maze.launch):

both launch file and world file are in the same directory and I've set the GAZEBOMODELPATH to that directory. Can someone tell me what's the problem here? I'm using Ubuntu 14.04, Indigo, and gazebo_ros.

Thanks