Fixes the problem with Dynmap disabling Stargate

src/main/java/net/knarcraft/stargate/config/DynmapManager.java

@@ -131,6 +131,14 @@ public final class DynmapManager {
         }
     }
 
+    /**
+     * Disables Dynmap integration
+     */
+    public static void disable() {
+        markerSet = null;
+        portalIcon = null;
+    }
+
     /**
      * Gets the id used for the given portal's marker
      *

src/main/java/net/knarcraft/stargate/config/StargateConfig.java

@@ -112,15 +112,21 @@ public final class StargateConfig {
         setupVaultEconomy();
 
         //Set up dynmap
-        DynmapAPI dynmapAPI = (DynmapAPI) Bukkit.getPluginManager().getPlugin("dynmap");
-        if (dynmapAPI != null) {
-            try {
-                DynmapManager.initialize(dynmapAPI);
-                DynmapManager.addAllPortalMarkers();
-            } catch (NullPointerException exception) {
-                logger.warning("Dynmap started in an invalid state. Check your log/console for dynmap-related " +
-                        "problems. Dynmap integration cannot be initialized.");
+        try {
+            DynmapAPI dynmapAPI = (DynmapAPI) Bukkit.getPluginManager().getPlugin("dynmap");
+            if (dynmapAPI != null) {
+                try {
+                    DynmapManager.initialize(dynmapAPI);
+                    DynmapManager.addAllPortalMarkers();
+                } catch (NullPointerException ignored) {
+                    logger.warning("Dynmap started in an invalid state. Check your log/console for dynmap-related " +
+                            "problems. Dynmap integration cannot be initialized.");
+                }
             }
+        } catch (NoClassDefFoundError error) {
+            logger.warning("Dynmap seems to be unavailable, even though its API is registered. Dynmap " +
+                    "integration is disabled.");
+            DynmapManager.disable();
         }
 
         this.isLoaded = true;