Sorting fails if all Domain instances are the same

[spranger]

if there are only Speakers all named exactly the same (at least 2), the sorting fails with

Caused by GroovyCastException: Cannot cast object 'null' with class 'null' to class 'int'. Try 'java.lang.Integer' instead
->> 324 | countRunAndMakeAscending in java.util.TimSort

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| 189 | sort in ''
| 173 | sort . . in ''
| 659 | sort in java.util.Arrays
| 217 | sort . . in java.util.Collections