Hello,
you provided the `initial_values` and `initial_velocity` functions as lambda functions in your `Coefficients` class. You create an object of this class in the main function and then store a `SmartPointer` to it in an `OfflineData` object, which in turn is stored as a `SmartPointer` in a `TimeStep` object. You convert these lambda functions to the `dealii::Function` class with a helper function in "helper.h" (I assume). And for convenience you create references to these lambda functions over this chain of pointers. There seem to be plenty of possible sources for errors considering in all these interconnections.
Can you verify that your references in lines 1029-1030 are actually valid?
Best,
Marc