Restoring backup returns 500 if backup was not called before

[philippguertler]

Gentics Mesh Version, operating system, or hardware.

• v0.21.3

Problem

See title

Reproducer

• Start fresh Mesh instance
• Call this endpoint https://getmesh.io/docs/beta/raml/#admin_graphdb_restore_post

Expected behaviour and actual behaviour

I expected a 4xx status code with an appropriate error message.
Instead I got a 500 and this in the Mesh log:

11:41:17.966 [Misty Piplup] ERROR [vert.x-eventloop-thread-4] - Error for request in path: /api/v1/admin/graphdb/restore
11:41:17.966 [Misty Piplup] ERROR [vert.x-eventloop-thread-4] - Error:
java.lang.NullPointerException: null
	at java.util.Objects.requireNonNull(Objects.java:203) ~[na:1.8.0_172]
	at java.util.Arrays$ArrayList.<init>(Arrays.java:3813) ~[na:1.8.0_172]
	at java.util.Arrays.asList(Arrays.java:3800) ~[na:1.8.0_172]
	at com.gentics.mesh.core.endpoint.admin.AdminHandler.lambda$handleRestore$5(AdminHandler.java:93) ~[classes/:na]
	at com.gentics.mesh.graphdb.spi.Database.lambda$null$3(Database.java:121) ~[classes/:na]
	at io.vertx.core.impl.ContextImpl.lambda$executeBlocking$1(ContextImpl.java:273) ~[vertx-core-3.5.2.jar:3.5.2]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[na:1.8.0_172]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[na:1.8.0_172]
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.19.Final.jar:4.1.19.Final]
	at java.lang.Thread.run(Thread.java:748) ~[na:1.8.0_172]
11:41:17.967 [Misty Piplup] ERROR [vert.x-eventloop-thread-4] - 127.0.0.1 - - [Tue, 10 Jul 2018 09:41:17 GMT] "POST /api/v1/admin/graphdb/restore HTTP/1.1" 500 3 "-" "-"

This line is the issue:

mesh/core/src/main/java/com/gentics/mesh/core/endpoint/admin/AdminHandler.java

Line 93 in 8c1ca01

File latestFile = Arrays.asList(backupDir.listFiles()).stream().filter(file -> file.getName().endsWith(".zip"))

listFiles() returns null if the directory cannot be found.