Fix backwards compat with nullable classes

Fixes #499

modules/API/src/main/java/com/comphenix/protocol/utility/CachedPackage.java

@@ -63,21 +63,16 @@ class CachedPackage {
 	 * @return Class object.
 	 * @throws RuntimeException If we are unable to find the given class.
 	 */
-	public Optional<Class<?>> getPackageClass(String className) {
+	public Optional<Class<?>> getPackageClass(final String className) {
 		Preconditions.checkNotNull(className, "className cannot be null!");
 
-		Optional<Class<?>> result = cache.get(className);
-		if (result == null) {
+		return cache.computeIfAbsent(className, x -> {
 			try {
-				Class<?> clazz = source.loadClass(combine(packageName, className));
-				result = Optional.ofNullable(clazz);
-				cache.put(className, result);
+				return Optional.ofNullable(source.loadClass(combine(packageName, className)));
 			} catch (ClassNotFoundException ex) {
-				cache.put(className, Optional.empty());
+				return Optional.empty();
 			}
-		}
-
-		return result;
+		});
 	}
 
 	/**

modules/API/src/main/java/com/comphenix/protocol/utility/MinecraftReflection.java

@@ -2021,7 +2021,7 @@ public class MinecraftReflection {
 				.orElseThrow(() -> new RuntimeException("Failed to find NMS class: " + className));
 	}
 
-	private static Class<?> getNullableNMS(String className) {
+	static Class<?> getNullableNMS(String className) {
 		if (minecraftPackage == null)
 			minecraftPackage = new CachedPackage(getMinecraftPackage(), getClassSource());
 		return minecraftPackage.getPackageClass(className).orElse(null);

modules/ProtocolLib/src/test/java/com/comphenix/protocol/utility/MinecraftReflectionTest.java

@@ -1,8 +1,7 @@
 package com.comphenix.protocol.utility;
 
 import static com.comphenix.protocol.utility.TestUtils.assertItemsEqual;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.*;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
@@ -75,6 +74,11 @@ public class MinecraftReflectionTest {
 		MinecraftReflection.getBukkitEntity("Hello");
 	}
 
+	@Test
+	public void testNullable() {
+		assertNull(MinecraftReflection.getNullableNMS("ProtocolLib"));
+	}
+
 	@Test
 	public void testAttributeSnapshot() {
 		assertEquals(AttributeSnapshot.class, MinecraftReflection.getAttributeSnapshotClass());