Some NullPointerException

I get this exception only sometimes:
[java]
SEVERE: Uncaught exception thrown in Thread[LWJGL Renderer Thread,10,main]
java.lang.NullPointerException
at com.bulletphysics.collision.shapes.BvhTriangleMeshShape.processAllTriangles(BvhTriangleMeshShape.java:161)
at com.bulletphysics.collision.dispatch.ConvexConcaveCollisionAlgorithm.processCollision(ConvexConcaveCollisionAlgorithm.java:86)
at com.bulletphysics.collision.dispatch.CompoundCollisionAlgorithm.processCollision(CompoundCollisionAlgorithm.java:120)
at com.bulletphysics.collision.dispatch.DefaultNearCallback.handleCollision(DefaultNearCallback.java:55)
at com.bulletphysics.collision.dispatch.CollisionDispatcher$CollisionPairCallback.processOverlap(CollisionDispatcher.java:236)
at com.bulletphysics.collision.broadphase.HashedOverlappingPairCache.processAllOverlappingPairs(HashedOverlappingPairCache.java:190)
at com.bulletphysics.collision.dispatch.CollisionDispatcher.dispatchAllCollisionPairs(CollisionDispatcher.java:247)
at com.bulletphysics.collision.dispatch.CollisionWorld.performDiscreteCollisionDetection(CollisionWorld.java:150)
at com.bulletphysics.dynamics.DiscreteDynamicsWorld.internalSingleStepSimulation(DiscreteDynamicsWorld.java:378)
at com.bulletphysics.dynamics.DiscreteDynamicsWorld.stepSimulation(DiscreteDynamicsWorld.java:339)
at com.jme3.bullet.PhysicsSpace.update(PhysicsSpace.java:324)
at com.jme3.bullet.PhysicsSpace.update(PhysicsSpace.java:311)
at com.jme3.bullet.BulletAppState.render(BulletAppState.java:212)
at com.jme3.app.state.AppStateManager.render(AppStateManager.java:274)
at com.jme3.app.SimpleApplication.update(SimpleApplication.java:251)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.runLoop(LwjglAbstractDisplay.java:151)
at com.jme3.system.lwjgl.LwjglDisplay.runLoop(LwjglDisplay.java:185)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.run(LwjglAbstractDisplay.java:228)
at java.lang.Thread.run(Thread.java:662)
[/java]
Any idea why before I might dive into the source code?

Most often this happens when you add or remove objects from the physics space twice or you have invalid meshes (e.g. not triangles but strips or so).
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