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You can first try to check if the message is empty by using LaserScanMsg.ranges.empty(). If this returns a 1, then your message is indeed empty, and any other action you try to do on it will result in a Segmentation Fault.

I would suggest the following:

if( !LaserScanMsg.ranges.empty() ) { return LaserScanMsg.ranges[LaserScanMsg.ranges.size()/2]; }

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