When the parser sees no <inertial> tag inside a <deformable>, it logs an error but does not return false. So in the next step, a null pointer is dereferenced.
This can be tested with loading the following URDF:
```
<?xml version="1.0"?>
<robot name="rhythm_carrot_sticks">
<deformable name="bag">
</deformable>
</robot>
```
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