Fix issue #142

Add unit test for MobRepository#remove(null)

Add explicit null check

Remove empty line, change useless exception message

game/src/test/org/apollo/game/model/entity/MobRepositoryTest.java

@@ -73,6 +73,15 @@ public final class MobRepositoryTest {
 		assertEquals(0, players.size());
 	}
 
+	/**
+	 * Tests failing of {@link MobRepository#remove(Mob)}
+	 */
+	@Test(expected = NullPointerException.class)
+	public void testRemoveNull() {
+		MobRepository<Player> players = new MobRepository<>(CAPACITY);
+		players.remove(null);
+	}
+
 	/**
 	 * Ensures that a MobRepository maintains a fixed capacity.
 	 */