Use our own firstNonNull to maintain back-compat.

Fixes WORLDEDIT-3567.

worldedit-core/src/main/java/com/sk89q/worldedit/util/GuavaUtil.java

@@ -0,0 +1,33 @@
+/*
+ * WorldEdit, a Minecraft world manipulation toolkit
+ * Copyright (C) sk89q <http://www.sk89q.com>
+ * Copyright (C) WorldEdit team and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.sk89q.worldedit.util;
+
+import javax.annotation.Nullable;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+public final class GuavaUtil {
+
+    private GuavaUtil() {}
+
+    public static <T> T firstNonNull(@Nullable T first, @Nullable T second) {
+        return first != null ? first : checkNotNull(second);
+    }
+}