- Fix a rare NPE related to trying to show a card to a particular player outside of game context.

- Allow all cards to be shown the player when in developer mode (in order to be consistent with the fact that the player is able to see everything in dev mode including both players' hands and the content of both libraries).

forge-game/src/main/java/forge/game/card/Card.java

@@ -8869,6 +8869,8 @@ public class Card extends GameEntity implements Comparable<Card> {
     }
 
     public boolean canBeShownTo(final Player viewer) {
+        if (viewer == null) { return false; }
+
         Zone zone = this.getZone();
         if (zone == null) { return true; } //cards outside any zone are visible to all