Fix backend factory fallback before simulator initialization

Signed-off-by: Sylvester Kaczmarek <16242628+sylvesterkaczmarek@users.noreply.github.com>

Author: sylvesterkaczmarek

source/isaaclab/changelog.d/sylvesterkaczmarek-fix-backend-fallback.rst

@@ -0,0 +1,5 @@
+Fixed
+^^^^^
+
+* Fixed backend factory resolution raising before simulator initialization instead of using the documented
+  ``physx`` compatibility fallback when no ``SimulationContext`` exists.

source/isaaclab/isaaclab/utils/backend_utils.py

@@ -79,7 +79,11 @@ def _get_backend(cls, *args, **kwargs) -> str:
         # Import lazily to avoid import cycles at module load time.
         from isaaclab.sim.simulation_context import SimulationContext
 
-        manager_name = SimulationContext.instance().physics_manager.__name__.lower()
+        sim_context = SimulationContext.instance()
+        if sim_context is None:
+            return "physx"
+
+        manager_name = sim_context.physics_manager.__name__.lower()
         if manager_name.startswith("newton"):
             return "newton"
         if manager_name.startswith("ovphysx"):

source/isaaclab/test/utils/test_backend_fallback.py

@@ -0,0 +1,17 @@
+# Copyright (c) 2022-2026, The Isaac Lab Project Developers (https://github.com/isaac-sim/IsaacLab/blob/main/CONTRIBUTORS.md).
+# All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+
+import pytest
+
+from isaaclab.sim.simulation_context import SimulationContext
+from isaaclab.utils.backend_utils import FactoryBase
+
+pytestmark = pytest.mark.unit
+
+
+def test_factory_backend_falls_back_to_physx_without_simulation_context(monkeypatch):
+    monkeypatch.setattr(SimulationContext, "_instance", None)
+
+    assert FactoryBase._get_backend() == "physx"