Ocean Modifier: prevent crashing on 'Apply Modifier' button

The 'Apply Modifier' button calls the modifier code on the original
object instead of an evaluated copy, which doesn't have an initialised
Ocean *.

source/blender/blenkernel/BKE_ocean.h

@@ -74,6 +74,7 @@ typedef struct OceanCache {
 struct Ocean *BKE_ocean_add(void);
 void BKE_ocean_free_data(struct Ocean *oc);
 void BKE_ocean_free(struct Ocean *oc);
+bool BKE_ocean_ensure(struct OceanModifierData *omd);
 void BKE_ocean_init_from_modifier(struct Ocean *ocean, struct OceanModifierData const *omd);
 
 void BKE_ocean_init(

source/blender/blenkernel/intern/ocean.c

@@ -836,6 +836,17 @@ struct Ocean *BKE_ocean_add(void)
 	return oc;
 }
 
+bool BKE_ocean_ensure(struct OceanModifierData *omd)
+{
+	if (omd->ocean) {
+		return false;
+	}
+
+	omd->ocean = BKE_ocean_add();
+	BKE_ocean_init_from_modifier(omd->ocean, omd);
+	return true;
+}
+
 void BKE_ocean_init_from_modifier(struct Ocean *ocean, struct OceanModifierData const *omd)
 {
 	short do_heightfield, do_chop, do_normals, do_jacobian;
@@ -1530,6 +1541,11 @@ void BKE_ocean_bake(struct Ocean *UNUSED(o), struct OceanCache *UNUSED(och),
 	/* unused */
 	(void)update_cb;
 }
+
+void BKE_ocean_init_from_modifier(struct Ocean *UNUSED(ocean), struct OceanModifierData const *UNUSED(omd))
+{
+}
+
 #endif /* WITH_OCEANSIM */
 
 void BKE_ocean_free_modifier_cache(struct OceanModifierData *omd)

source/blender/modifiers/intern/MOD_ocean.c

@@ -380,6 +380,7 @@ static Mesh *doOcean(ModifierData *md, const ModifierEvalContext *ctx, Mesh *mes
 	OceanModifierData *omd = (OceanModifierData *) md;
 	int cfra_scene = (int)DEG_get_ctime(ctx->depsgraph);
 	Object *ob = ctx->object;
+	bool allocated_ocean = false;
 
 	Mesh *result = NULL;
 	OceanResult ocr;
@@ -410,6 +411,12 @@ static Mesh *doOcean(ModifierData *md, const ModifierEvalContext *ctx, Mesh *mes
 		BKE_ocean_simulate_cache(omd->oceancache, cfra_scene);
 	}
 	else {
+		/* omd->ocean is NULL on an original object (in contrast to an evaluated one).
+		 * We can create a new one, but we have to free it as well once we're done.
+		 * This function is only called on an original object when applying the modifier
+		 * using the 'Apply Modifier' button, and thus it is not called frequently for
+		 * simulation. */
+		allocated_ocean |= BKE_ocean_ensure(omd);
 		simulate_ocean_modifier(omd);
 	}
 
@@ -503,6 +510,11 @@ static Mesh *doOcean(ModifierData *md, const ModifierEvalContext *ctx, Mesh *mes
 		}
 	}
 
+	if (allocated_ocean) {
+		BKE_ocean_free(omd->ocean);
+		omd->ocean = NULL;
+	}
+
 #undef OCEAN_CO
 
 	return result;