[Bug]:App crahes when the layout is empty and you try to get angle properties

I have opened a PR regrading this bug .

I think all assert statements in graph.cpp has to be replaced by something approapriate it lead to issues in future

Thanks. I left some comments on the pull request. I’d make sure to also change the calling code (e.g., angle iterator, dihedral iterator, property model) to check for empty molecules, molecules without enough atoms, etc.