How to fix: REQUIRED process [sitl-1] has died! process has died

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I have succesfully installed mavros and px4. On doing [make px4_sitl_defaul gazebo], I got the initial gazebo simulation and stuff like commander takeoff and land working just fine. But later on, upon [roslaunch px4 mavros_posix_sitl.launch], the above error pops up and gazebo does not start. I have cross checked the .bashrc and setup.bash file of my workspace for any path or sourcing errors. Can someone please help me with this issue. Its recurring even after multiple [catkin_build] AND installation of mavros and px4 from scratch.