Avoid attempting to fetch a non-managed RealmLive instance from the realm backing

For compatibility reasons, we quite often convert completely unmanaged
instances to `ILive`s so they fit the required parameters of a property
or method call. This ensures such cases will not cause any issues when
trying to interact with the underlying data.

Originally I had this allowing write operations, but that seems a bit
unsafe (when performing a write one would assume that the underlying
data is being persisted, whereas in this case it is not). We can change
this if the requirements change in the future, but I think throwing is
the safest bet for now.

osu.Game.Tests/Database/RealmLiveTests.cs

@@ -30,6 +30,23 @@ namespace osu.Game.Tests.Database
             });
         }
 
+        [Test]
+        public void TestAccessNonManaged()
+        {
+            var beatmap = new RealmBeatmap(CreateRuleset(), new RealmBeatmapDifficulty(), new RealmBeatmapMetadata());
+            var liveBeatmap = beatmap.ToLive();
+
+            Assert.IsFalse(beatmap.Hidden);
+            Assert.IsFalse(liveBeatmap.Value.Hidden);
+            Assert.IsFalse(liveBeatmap.PerformRead(l => l.Hidden));
+
+            Assert.Throws<InvalidOperationException>(() => liveBeatmap.PerformWrite(l => l.Hidden = true));
+
+            Assert.IsFalse(beatmap.Hidden);
+            Assert.IsFalse(liveBeatmap.Value.Hidden);
+            Assert.IsFalse(liveBeatmap.PerformRead(l => l.Hidden));
+        }
+
         [Test]
         public void TestValueAccessWithOpenContext()
         {