GuiChoose invokes ListChooser from EDT thread.

src/main/java/forge/FThreads.java

@@ -15,6 +15,14 @@ import forge.control.input.InputSynchronized;
  *
  */
 public class FThreads {
+    
+    // This could be some bad copy of SwingWorker
+    public abstract static class RunnableWithResult<T> implements Runnable {
+        protected T result = null;
+        public T getResult() { return result; }
+
+    }
+    
     static { 
         System.out.printf("(FThreads static ctor): Running on a machine with %d cpu core(s)%n", Runtime.getRuntime().availableProcessors() );
     }

src/main/java/forge/control/input/InputPayManaBase.java

@@ -14,7 +14,7 @@ import forge.card.mana.ManaPool;
 import forge.card.spellability.AbilityManaPart;
 import forge.card.spellability.SpellAbility;
 import forge.game.GameState;
-import forge.game.player.Player;
+import forge.game.player.HumanPlayer;
 import forge.game.zone.ZoneType;
 import forge.gui.GuiChoose;
 import forge.gui.framework.SDisplayUtil;
@@ -291,8 +291,7 @@ public abstract class InputPayManaBase extends InputSyncronizedBase implements I
         Runnable proc = new Runnable() {
             @Override
             public void run() {
-                final Player p = chosen.getActivatingPlayer();
-                p.getGame().getActionPlay().playSpellAbility(chosen, p);
+                player.getGame().getActionPlay().playSpellAbility(chosen, (HumanPlayer)chosen.getActivatingPlayer());
                 onManaAbilityPlayed(chosen); 
             }
         };

src/main/java/forge/gui/GuiChoose.java

@@ -7,6 +7,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
+import java.util.concurrent.Callable;
 
 import javax.swing.JDialog;
 import javax.swing.JFrame;
@@ -16,6 +17,7 @@ import javax.swing.event.ListSelectionEvent;
 import javax.swing.event.ListSelectionListener;
 
 import forge.Card;
+import forge.FThreads;
 import forge.gui.match.CMatchUI;
 import forge.item.InventoryItem;
 
@@ -102,6 +104,10 @@ public class GuiChoose {
             }
         }
         
+        FThreads.RunnableWithResult<List<T>> showChoice = new FThreads.RunnableWithResult<List<T>>() {
+            
+            @Override
+            public void run() {
                 ListChooser<T> c = new ListChooser<T>(message, min, max, choices);
                 final JList list = c.getJList();
                 list.addListSelectionListener(new ListSelectionListener() {
@@ -120,7 +126,12 @@ public class GuiChoose {
                 });
                 c.show();
                 GuiUtils.clearPanelSelections();
-        return c.getSelectedValues();
+                result = c.getSelectedValues();
+            }
+        };
+
+        FThreads.invokeInEDTAndWait(showChoice);
+        return showChoice.getResult();
     }
 
     // Nothing to choose here. Code uses this to just show a card.