[Bug] URDF Importer crashes with 'Used null prim' on URDFs with empty base_footprint root link

[xteo]

Description

The URDF importer (URDFParseAndImportFile) crashes with RuntimeError: Used null prim when importing URDFs that have an empty root link (e.g., base_footprint) — a very common pattern in ROS robot descriptions.

Steps to Reproduce

1. Use Isaac Sim 6.0.0-rc.13 (built from source on ARM64/GB300)
2. Import any URDF with an empty root link pattern:

<link name="base_footprint"/>
<joint name="base_joint" type="fixed">
  <parent link="base_footprint"/>
  <child link="base_link"/>
</joint>

3. Both merge_fixed_joints=True and merge_fixed_joints=False crash

Error Output

[Warning] [isaacsim.asset.importer.urdf] Link base_footprint has no colliders, and no inertia was imported; assigning a small isotropic inertia matrix
[Warning] [omni.usd] Coding Error: in _IsValidPathForCreatingPrim at line 3720 -- Path must be an absolute path: <>
[Error] Failed to execute a command: URDFParseAndImportFile.
RuntimeError: Used null prim

Expected Behavior

The importer should handle empty root links gracefully (skip them, merge into the first child with geometry, or treat as the articulation root without geometry).

Environment

• Isaac Sim 6.0.0-rc.13 (built from develop branch)
• Platform: ARM64 (NVIDIA DGX Station GB300)
• CUDA 13.1 / Driver 590.48.01
• Ubuntu 24.04 (Noble Numbat)

Workaround

Convert STL meshes to USD manually using omni.kit.asset_converter, then assemble the robot in USD with correct transforms from the URDF. Bypasses the URDF importer entirely.

Affected Robot

MiR250 (Mobile Industrial Robots) — uses standard ROS base_footprint → base_link pattern. The DFKI-NI/mir_robot ROS package URDF fails to import.

[vick-yu]

Thanks for the clear report and repro steps. Could you check whether this still reproduces in Isaac Sim 6.0-rc.22 (current repo version)?