fix NPE in bulk item filter where holding module has no target

#256

Author: desht

src/main/java/me/desht/modularrouters/client/gui/filter/BulkItemFilterScreen.java

@@ -62,7 +62,7 @@ public void init() {
             ModularRouterBlockEntity router = menu.getRouter();
             CompiledModule cm = ((ModuleItem) moduleStack.getItem()).compile(router, moduleStack);
             target = cm.getEffectiveTarget(router);
-            if (target.hasItemHandler(Minecraft.getInstance().level)) {
+            if (target != null && target.hasItemHandler(Minecraft.getInstance().level)) {
                 MutableComponent title = xlate(target.blockTranslationKey);
                 addRenderableWidget(new MergeButton(leftPos + 28, topPos + 130, target.toString(), title, p -> {
                     if (target != null) {