Fix vanishing "Global Options" property when adding a RobotModelDisplay

[rhaschke]

With Qt 5.15, the Global Options property was vanishing, when a RobotModelDisplay was added with links having no geometry. In this case, the Alpha property of the corresponding link is hidden.
However, hiding a property:

rviz/src/rviz/properties/property.cpp

Line 398 in 4129ee4

model_->emitPropertyHiddenChanged(this);

during the process of reparenting:

rviz/src/rviz/properties/property.cpp

Line 373 in 4129ee4

child->setModel(model_);

caused setRowHidden to be called with an invalid parentIndex:

rviz/src/rviz/properties/property_tree_widget.cpp

Line 125 in 4129ee4

setRowHidden(property->rowNumberInParent(), model_->parentIndex(property), property->getHidden());

which then caused hiding of the corresponding top-level property.