Fixed NPE when opening the Settings Panel with some flags being hidden

The NPE could only occur if a non-op player was trying to open the Settings Panel or change tab or change mode, leading to the call of a null PanelItem.

I added some nullability annotations to make it 100% clear that Flag#toPanelItem(...) returns null if the player is not op and the flag is invisible.

src/main/java/world/bentobox/bentobox/api/flags/Flag.java

@@ -359,6 +359,7 @@ public class Flag implements Comparable<Flag> {
      * @param invisible - true if this flag is not visible to players
      * @return - PanelItem for this flag or null if item is invisible to user
      */
+    @Nullable
     public PanelItem toPanelItem(BentoBox plugin, User user, @Nullable Island island, boolean invisible) {
         // Invisibility
         if (!user.isOp() && invisible) {

src/main/java/world/bentobox/bentobox/api/panels/Tab.java

@@ -1,5 +1,7 @@
 package world.bentobox.bentobox.api.panels;
 
+import org.eclipse.jdt.annotation.Nullable;
+
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
@@ -25,7 +27,7 @@ public interface Tab {
      * Return the panel items for this tab
      * @return a list of items in slot order
      */
-    List<PanelItem> getPanelItems();
+    List<@Nullable PanelItem> getPanelItems();
 
     /**
      * @return the permission required to view this tab or empty if no permission required

src/main/java/world/bentobox/bentobox/api/panels/TabbedPanel.java

@@ -3,6 +3,7 @@ package world.bentobox.bentobox.api.panels;
 import java.security.InvalidParameterException;
 import java.util.List;
 import java.util.Map.Entry;
+import java.util.Objects;
 import java.util.TreeMap;
 
 import org.bukkit.Material;
@@ -88,7 +89,7 @@ public class TabbedPanel extends Panel implements PanelListener {
         // Show the active tab
         if (tpb.getTabs().containsKey(activeTab)) {
             List<PanelItem> panelItems = tab.getPanelItems();
-            panelItems.stream().skip(page * 43L).limit(page * 43L + 43L).forEach(i -> items.put(items.lastKey() + 1, i));
+            panelItems.stream().filter(Objects::nonNull).skip(page * 43L).limit(page * 43L + 43L).forEach(i -> items.put(items.lastKey() + 1, i));
             // Add forward and backward icons
             if (page > 0) {
                 // Previous page icon

src/main/java/world/bentobox/bentobox/panels/settings/SettingsTab.java

@@ -11,6 +11,8 @@ import org.bukkit.Sound;
 import org.bukkit.World;
 import org.bukkit.event.inventory.ClickType;
 
+import org.eclipse.jdt.annotation.NonNull;
+import org.eclipse.jdt.annotation.Nullable;
 import world.bentobox.bentobox.BentoBox;
 import world.bentobox.bentobox.api.flags.Flag;
 import world.bentobox.bentobox.api.flags.Flag.Type;
@@ -111,7 +113,8 @@ public class SettingsTab implements Tab, ClickHandler {
      * @return list of all the panel items for this flag type
      */
     @Override
-    public List<PanelItem> getPanelItems() {
+    @NonNull
+    public List<@Nullable PanelItem> getPanelItems() {
         List<Flag> flags = getFlags();
         int i = 0;
         // Jump past empty tabs