Avoid addGreatPerson exception.

core/src/com/unciv/logic/civilization/CivilizationInfo.kt

@@ -350,10 +350,14 @@ class CivilizationInfo {
             city.endTurn()
         }
 
+        //if no city available, addGreatPerson will throw exception
+        if (!cities.isNotEmpty()) {
             val greatPerson = greatPeople.getNewGreatPerson()
             if (greatPerson != null) {
                 addGreatPerson(greatPerson)
             }
+        }
+
 
         goldenAges.endTurn(happiness)
         getCivUnits().forEach { it.endTurn() }