report change after call Design.saveAs() in 251?

Migrateduser

my application met an "The context can not be null when creating a structure handle!" error when i updated birt engine 232 to 251 (stable released on 9.26), i went deep in source code and found this error is throwed by StructureHandler. the root cause for this error is there is an structure whose context is null, but this structure is not added by my application, i debugged my application and found this null structure is added after Design.saveAs()

following are my debug code for this issue
Object a = design.getBody().get(0);
design.saveAs("c:/report251.rptdesign");
a = design.getBody().get(0);
object a is changed after invoking Design.saveAs(), and runAndRender this design file will throw "The context can not be null when creating a structure handle!" error, but if i remove Design.saveAs(), the report can run and render successfully. anyone met same issue before?

also i notice that only report which has group have this issue and the new added struture is related with group and added in table's binding column whose name is like column_1.