Fix T80596: Convert to Curve from Mesh crashes Blender

The point cache code needs a non NULL rbw pointer.

This could have been avoided if there was a sanity check in the convert
function, so added a check there as well.

source/blender/blenkernel/intern/pointcache.c

@@ -1848,7 +1848,7 @@ void BKE_ptcache_id_from_rigidbody(PTCacheID *pid, Object *ob, RigidBodyWorld *r
 
   memset(pid, 0, sizeof(PTCacheID));
 
-  pid->owner_id = &ob->id;
+  pid->owner_id = ob != NULL ? &ob->id : NULL;
   pid->calldata = rbw;
   pid->type = PTCACHE_TYPE_RIGIDBODY;
   pid->cache = rbw->shared->pointcache;

source/blender/blenkernel/intern/rigidbody.c

@@ -1554,18 +1554,18 @@ void BKE_rigidbody_remove_object(Main *bmain, Scene *scene, Object *ob, const bo
       BKE_collection_object_add(bmain, scene->master_collection, ob);
     }
     BKE_collection_object_remove(bmain, rbw->group, ob, free_us);
+
+    /* flag cache as outdated */
+    BKE_rigidbody_cache_reset(rbw);
+    /* Reset cache as the object order probably changed after freeing the object. */
+    PTCacheID pid;
+    BKE_ptcache_id_from_rigidbody(&pid, NULL, rbw);
+    BKE_ptcache_id_reset(scene, &pid, PTCACHE_RESET_OUTDATED);
   }
 
   /* remove object's settings */
   BKE_rigidbody_free_object(ob, rbw);
 
-  /* flag cache as outdated */
-  BKE_rigidbody_cache_reset(rbw);
-  /* Reset cache as the object order probably changed after freeing the object. */
-  PTCacheID pid;
-  BKE_ptcache_id_from_rigidbody(&pid, NULL, rbw);
-  BKE_ptcache_id_reset(scene, &pid, PTCACHE_RESET_OUTDATED);
-
   /* Dependency graph update */
   DEG_relations_tag_update(bmain);
   DEG_id_tag_update(&ob->id, ID_RECALC_TRANSFORM);

source/blender/editors/object/object_add.c

@@ -2572,7 +2572,9 @@ static int object_convert_exec(bContext *C, wmOperator *op)
 
       if (newob->type == OB_CURVE) {
         BKE_object_free_modifiers(newob, 0); /* after derivedmesh calls! */
-        ED_rigidbody_object_remove(bmain, scene, newob);
+        if (newob->rigidbody_object != NULL) {
+          ED_rigidbody_object_remove(bmain, scene, newob);
+        }
       }
     }
     else if (ob->type == OB_MESH && target == OB_GPENCIL) {