Items with type topup have a price = null and hence not even the property set (ignored in JSON). This needs to be handled in the temlate, otherwise this exception occurs:
```
An unhandled exception was thrown by the application.
System.InvalidOperationException: Nullable object must have a value.
at AspNetCoreGeneratedDocument.Views_Shared_Crowdfund_Public_ContributeForm.<>c__DisplayClass24_0.<<ExecuteAsync>b__0>d.MoveNext()
```
