Fix: Lecterns not opening when there is no lectern cache (#4476)

... the LecternCache doesn't always exist. Oops.

core/src/main/java/org/geysermc/geyser/session/GeyserSession.java

@@ -285,7 +285,7 @@ public class GeyserSession implements GeyserConnection, GeyserCommandSource {
      * See {@link WorldManager#sendLecternData(GeyserSession, int, int, int)}
      * for more information.
      */
-    private final Set<Vector3i> lecternCache;
+    private final @Nullable Set<Vector3i> lecternCache;
 
     /**
      * A list of all players that have a player head on with a custom texture.

core/src/main/java/org/geysermc/geyser/translator/inventory/LecternInventoryTranslator.java

@@ -193,9 +193,11 @@ public class LecternInventoryTranslator extends AbstractBlockInventoryTranslator
             lecternContainer.setPosition(position);
 
             BlockEntityUtils.updateBlockEntity(session, blockEntityTag, position);
-            session.getLecternCache().add(position);
 
             if (shouldRefresh) {
+                // the lectern cache doesn't always exist; only when we must refresh
+                session.getLecternCache().add(position);
+
                 // Close the window - we will reopen it once the client has this data synced
                 ServerboundContainerClosePacket closeWindowPacket = new ServerboundContainerClosePacket(lecternContainer.getJavaId());
                 session.sendDownstreamGamePacket(closeWindowPacket);