Fix a crash in the grid_config plugin

[nkoenig]

The grid plugin will crash when changing properties if a world has <grid>false</grid> in the <scene> tag.

For example:

<?xml version="1.0" ?>
<sdf version="1.6">
  <world name="default">
    <plugin
      filename="ignition-gazebo-user-commands-system"
      name="ignition::gazebo::systems::UserCommands">
    </plugin>
    <plugin
      filename="ignition-gazebo-scene-broadcaster-system"
      name="ignition::gazebo::systems::SceneBroadcaster">
    </plugin>
    <physics name="1ms" type="ode">
      <max_step_size>0.004</max_step_size>
      <real_time_factor>1.0</real_time_factor>
    </physics>

    <scene>
      <ambient>0.1 0.1 0.1 1.0</ambient>
      <background>0 0 0 1.0</background>
      <grid>false</grid>
      <origin_visual>false</origin_visual>
    </scene>

    <include>
      <static>true</static>
      <name>staging_area</name>
      <pose>0 0 0 0 0 0</pose>
      <uri>https://fuel.ignitionrobotics.org/1.0/OpenRobotics/models/Cave Starting Area Type B</uri>
    </include>
  </world>
</sdf>

This fixes the problem, but there might be a more elegant solution.

[chapulina]

there might be a more elegant solution.

See #413

[codecov]

Codecov Report

Merging #412 into ign-gazebo3 will increase coverage by 11.59%.
The diff coverage is 80.21%.

@@               Coverage Diff                @@
##           ign-gazebo3     #412       +/-   ##
================================================
+ Coverage        65.71%   77.30%   +11.59%     
================================================
  Files              127      205       +78     
  Lines             6238    11018     +4780     
================================================
+ Hits              4099     8518     +4419     
- Misses            2139     2500      +361     

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
include/ignition/gazebo/Link.hh	100.00% <ø> (ø)
include/ignition/gazebo/Model.hh	100.00% <ø> (ø)
include/ignition/gazebo/SdfEntityCreator.hh	100.00% <ø> (ø)
include/ignition/gazebo/Server.hh	100.00% <ø> (ø)
include/ignition/gazebo/ServerConfig.hh	100.00% <ø> (ø)
include/ignition/gazebo/gui/GuiEvents.hh	0.00% <0.00%> (ø)
src/SimulationRunner.hh	100.00% <ø> (ø)
.../plugins/component_inspector/ComponentInspector.hh	28.57% <ø> (ø)
src/gui/plugins/entity_tree/EntityTree.cc	9.89% <0.00%> (ø)
... and 171 more

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