ConcurrentModification error: slightly hacky fix (#12791)

* ConcurrentModification error: slightly hacky fix

Should be a fix for #12789
This is not an ideal fix, and we likely have this problem in other areas.
It is possible this is a non-fix too, we should watch out for further error
reports to check carefully if this was a non-fix.

* Formatting

game-app/game-headed/src/main/java/games/strategy/triplea/ui/PlacePanel.java

@@ -22,6 +22,7 @@
 import java.awt.Dimension;
 import java.awt.Toolkit;
 import java.awt.event.MouseEvent;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.List;
@@ -145,15 +146,23 @@ private void updateStep() {
       }
     }
 
-    SwingUtilities.invokeLater(
-        () -> {
-          if (showUnitsToPlace) {
-            unitsToPlacePanel.setUnits(unitsToPlace);
+    if (showUnitsToPlace) {
+      // Small hack: copy the unit list before passing it to a new thread.
+      // This is to prevent ConcurrentModification. If the 'unitsToPlace' list is modified
+      // later in this thread, before "SwingUtilities.invokeLater" can execute and complete,
+      // then we will get a ConcurrentModification exception.
+      // Ideally we would not modify the 'unitsToPlace' collection again except when
+      // the swing thread signals that the user has taken action.. Short of that, we create a copy
+      // here.
+      final Collection<Unit> unitsToPlaceCopy = new ArrayList<>(unitsToPlace);
+      SwingUtilities.invokeLater(
+          () -> {
+            unitsToPlacePanel.setUnits(unitsToPlaceCopy);
             SwingComponents.redraw(unitsToPlacePanel);
-          } else {
-            unitsToPlacePanel.removeAll();
-          }
-        });
+          });
+    } else {
+      SwingUtilities.invokeLater(unitsToPlacePanel::removeAll);
+    }
   }
 
   @Override