Fix for sensor pointer null when navsat plugin in included in sdf

[ashBabu]

🦟 Bug fix

Fixes #2163

Summary

When the play button is pressed in Gazebo, the terminal window shows an error as described in #2163.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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Test it

• create a model.sdf with the following navsat plugin

<sensor name="navsat" type="navsat">
   <navsat>
        ....
   </navsat>
   <plugin  filename="libignition-gazebo-navsat-system"  
            name="ignition::gazebo::systems::NavSat">
   </plugin>
</sensor>

• ign gazebo model.sdf
• Press the play button
• There wont be any error on the terminal

[mjcarroll]

Minor formatting issues.

Can you also update the title of the PR?

[ashBabu]

Minor formatting issues.

Can you also update the title of the PR?

done

[ashBabu]

@mjcarroll : I am not sure why some of the checks were unsuccessful. Is there something that I could do to get this approved? Thanks

[mjcarroll]

I am not sure why some of the checks were unsuccessful.

I'm still seeing the code not wrapped at 80 characters, that may be triggering a lint failure.

I believe the other failures are related to #2190, so you won't need to fix them.

[mjcarroll]

@osrf-jenkins retest this please

[iche033]

just a minor typo otherwise looks good to me