Don't fail on product without placement

src/ifcgeom/mapping/IfcProduct.cpp

@@ -8,7 +8,12 @@ taxonomy::ptr mapping::map_impl(const IfcSchema::IfcProduct* inst) {
 	// @todo decide on this, what happens in the product mapping?
 	// currently things like openings, layers and materials are processed in the converter
 	auto c = taxonomy::make<taxonomy::collection>();
-	c->matrix = taxonomy::cast<taxonomy::matrix4>(map(inst->ObjectPlacement()));
+	if (inst->ObjectPlacement()) {
+		c->matrix = taxonomy::cast<taxonomy::matrix4>(map(inst->ObjectPlacement()));
+	} else {
+		// @todo Otherwise we get crashes in the serializer, but maybe fix them there..?
+		c->matrix = taxonomy::make<taxonomy::matrix4>();
+	}
 	return c;
 
 	/*