Fix crash when loading converted Age.

Sources/Plasma/PubUtilLib/plPhysX/plSimulationMgr.cpp

@@ -650,7 +650,10 @@ int plSimulationMgr::GetMaterialIdx(NxScene* scene, float friction, float restit
     desc.dynamicFriction = friction;
     desc.staticFriction = friction;
     NxMaterial* mat = scene->createMaterial(desc);
+    if (mat)
         return mat->getMaterialIndex();
+    else
+        return NULL;
 }
 
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