Handle null container file for 'minecraft' on 1.13.x+

DynmapCore/src/main/java/org/dynmap/hdmap/TexturePack.java

@@ -1598,7 +1598,7 @@ public class TexturePack {
         // Check mods to see if texture files defined there
         for (String modid : core.getServer().getModList()) {
             File f = core.getServer().getModContainerFile(modid);   // Get mod file
-            if (f.isFile()) {
+            if ((f != null) && f.isFile()) {
                 ZipFile zf = null;
                 in = null;
                 try {