The message box above should not appear when using sensors. If it appears, there is maybe a problem with the acquisition on the USB. Do you see any warnings/errors on the terminal? The problem comes from
this method, which seems to return null data. Is your Xtion plugged in a USB 3 or USB 2?
You could also try to install OpenNI2 to see if could work instead of OpenNI. If you have
ROS, you can simply install openni2 like that:
sudo apt-get install ros-indigo-openni2-camera
then build again rtabmap.
cheers