Fix backend factory fallback before simulator initialization (isaac-sim#7121)

# Description

`FactoryBase._get_backend()` can be called before a `SimulationContext`
has been created, but it previously dereferenced
`SimulationContext.instance().physics_manager` unconditionally.
`SimulationContext.instance()` returns `None` before initialization, so
backend resolution could fail before any fallback was applied.

Following maintainer confirmation, the pre-context fallback is now
`newton`. Initialized backend resolution remains unchanged for Newton,
PhysX, and OV PhysX contexts.

A separate follow-up should update `SimulationCfg(physics=None)` to
default to Newton as well; that broader configuration-default change is
intentionally outside this PR.

## Validation

Adds unit coverage for backend resolution with
`SimulationContext._instance` unset and verifies the `newton` fallback.

## Type of change

- Bug fix

---------

Signed-off-by: Sylvester Kaczmarek <16242628+sylvesterkaczmarek@users.noreply.github.com>
Co-authored-by: ooctipus <zhengyuz@nvidia.com>
(cherry picked from commit 9f65e3d)

Author: sylvesterkaczmarek

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

@@ -0,0 +1,5 @@
+Fixed
+^^^^^
+
+* Fixed backend factory resolution raising before simulator initialization by using the team-confirmed ``newton``
+  fallback when no ``SimulationContext`` exists.

source/isaaclab/isaaclab/utils/backend_utils.py

@@ -73,13 +73,16 @@ def register(cls, name: str, sub_class) -> None:
     def _get_backend(cls, *args, **kwargs) -> str:
         """Return active backend name for this factory.
 
-        Falls back to ``"physx"`` for backward compatibility when no simulation
-        context is initialized yet.
+        Falls back to ``"newton"`` when no simulation context is initialized yet.
         """
         # 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 "newton"
+
+        manager_name = sim_context.physics_manager.__name__.lower()
         if manager_name.startswith("newton"):
             return "newton"
         if manager_name.startswith("ovphysx"):

source/isaaclab/isaaclab/visualizers/base_visualizer.py

@@ -153,8 +153,11 @@ def physics_backend(self) -> str | None:
             Backend name string, or ``None`` when no simulation context is active yet.
         """
         try:
+            from isaaclab.sim.simulation_context import SimulationContext
             from isaaclab.utils.backend_utils import FactoryBase
 
+            if SimulationContext.instance() is None:
+                return None
             return FactoryBase._get_backend()
         except Exception:
             return None

source/isaaclab/test/utils/test_backend_utils.py

@@ -5,6 +5,7 @@
 
 """Tests for backend module resolution."""
 
+from isaaclab.sim.simulation_context import SimulationContext
 from isaaclab.utils.backend_utils import FactoryBase
 
 
@@ -24,3 +25,10 @@ def test_get_module_name_preserves_other_backend_conventions(monkeypatch):
     assert FactoryBase._get_package_name("newton") == "isaaclab_newton"
     assert FactoryBase._get_module_name("physx") == "isaaclab_physx.assets.articulation"
     assert FactoryBase._get_module_name("newton") == "isaaclab_newton.assets.articulation"
+
+
+def test_factory_backend_falls_back_to_newton_without_simulation_context(monkeypatch):
+    """Backend resolution uses Newton before a simulation context exists."""
+    monkeypatch.setattr(SimulationContext, "_instance", None)
+
+    assert FactoryBase._get_backend() == "newton"