Graph builder not working when reopened

I have made this graph builder and run it properly. I have saved this. When I reopened again it is not working. Following error is displayed. all the bandmath tabs are displaying to edit the equations.
java.lang.string cannot be cast to org.esa.snap.core.gpf.common.BandmathsOp$BandDescriptor

Looking for help. Thanking you.

I’m not sure about the bandmaths error but, you need to have only one directed acyclic graph. Here you have two.
Two separate graphs will not work.