Added defensive code to handle island adding to grid.

When an island cannot be added to the grid, it should show some helpful
console error.

Related to https://github.com/BentoBoxWorld/bentobox/issues/326

src/main/java/world/bentobox/bentobox/managers/island/IslandGrid.java

@@ -3,6 +3,7 @@ package world.bentobox.bentobox.managers.island;
 import java.util.Map.Entry;
 import java.util.TreeMap;
 
+import world.bentobox.bentobox.BentoBox;
 import world.bentobox.bentobox.database.objects.Island;
 
 /**
@@ -22,6 +23,9 @@ class IslandGrid {
         if (grid.containsKey(island.getMinX())) {
             TreeMap<Integer, Island> zEntry = grid.get(island.getMinX());
             if (zEntry.containsKey(island.getMinZ())) {
+                BentoBox.getInstance().logError("Cannot add island to grid because there is an overlapping");
+                BentoBox.getInstance().logError("island already registered at this location:" + island.getCenter());
+                BentoBox.getInstance().logError("This is most likely caused by island distances changing mid-game, or an old database file");
                 return false;
             } else {
                 // Add island

src/main/java/world/bentobox/bentobox/managers/island/NewIsland.java

@@ -118,7 +118,11 @@ public class NewIsland {
         }
         // Add to the grid
         island = plugin.getIslands().createIsland(next, user.getUniqueId());
-        // Save the player so that if the server is reset weird things won't happen
+        if (island == null) {
+            plugin.logError("Failed to make island! Island could not be added to the grid.");
+            return;
+        }
+        // TODO: Save the player so that if the server is reset weird things won't happen?
 
         // Clear any old home locations (they should be clear, but just in case)
         plugin.getPlayers().clearHomeLocations(world, user.getUniqueId());

src/test/java/world/bentobox/bentobox/listeners/ObsidianToLavaTest.java

@@ -21,7 +21,6 @@ import org.bukkit.entity.Player;
 import org.bukkit.event.block.Action;
 import org.bukkit.event.player.PlayerEvent;
 import org.bukkit.event.player.PlayerInteractEvent;
-import org.bukkit.inventory.Inventory;
 import org.bukkit.inventory.ItemFactory;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.PlayerInventory;