value index 0 out of range (0 - -1) at attribute no 4 (d)

I am dealing with continuous features and discrete class_var, i.e.
```py
atts = ['a','b','c','d']
domain = Orange.data.Domain([Orange.feature.Continuous(x) for x in atts[:-1]] +
[Orange.feature.Discrete(atts[-1])])
data = Orange.data.Table(domain)
data.append([1,2,3,0])
```
However, it said "value index 0 out of range (0 - -1) at attribute no 4 (d)". Please help!

Start by running the reported Orange.data.Domain and Orange.data.Table snippet, then inspect how Table.append handles the discrete class_var and its value list. Confirm why the class variable has no valid values and define the expected behavior for appending the row, using the reproduced error as the regression case.

Written by the indexing model from the issue text.