Fix TryGetEntitySystem and GetEntitySystemOrNull throwing with uninitialized EntitySystemManager (space-wizards#6512)

Author: DrSmugleaf

Robust.Shared.IntegrationTests/GameObjects/EntitySystemManagerUninitializedTests.cs

@@ -0,0 +1,25 @@
+using NUnit.Framework;
+using Robust.Shared.GameObjects;
+
+namespace Robust.Shared.IntegrationTests.GameObjects;
+
+[TestFixture]
+public sealed class EntitySystemManagerUninitializedTests
+{
+    private class DummySystem : EntitySystem;
+
+    [Test]
+    public void TryDoesNotThrow()
+    {
+        var systems = new EntitySystemManager();
+
+        // These should not throw even with an uninitalized entity system manager
+        Assert.DoesNotThrow(() => systems.TryGetEntitySystem(out DummySystem? _));
+        Assert.DoesNotThrow(() => systems.TryGetEntitySystem(typeof(DummySystem), out _));
+        Assert.DoesNotThrow(() => systems.GetEntitySystemOrNull<DummySystem>());
+
+        Assert.That(systems.TryGetEntitySystem(out DummySystem? _), Is.False);
+        Assert.That(systems.TryGetEntitySystem(typeof(DummySystem), out _), Is.False);
+        Assert.That(systems.GetEntitySystemOrNull<DummySystem>(), Is.Null);
+    }
+}

Robust.Shared/GameObjects/EntitySystemManager.cs

@@ -84,6 +84,10 @@ public T GetEntitySystem<T>()
 
         public T? GetEntitySystemOrNull<T>() where T : IEntitySystem
         {
+            // ReSharper disable once ConditionIsAlwaysTrueOrFalseAccordingToNullableAPIContract
+            if (SystemDependencyCollection == null)
+                return default;
+
             SystemDependencyCollection.TryResolveType<T>(out var system);
             return system;
         }
@@ -126,7 +130,9 @@ public void Resolve<T1, T2, T3, T4>([NotNull] ref T1? instance1, [NotNull] ref T
         public bool TryGetEntitySystem<T>([NotNullWhen(true)] out T? entitySystem)
             where T : IEntitySystem
         {
-            return SystemDependencyCollection.TryResolveType<T>(out entitySystem);
+            entitySystem = default;
+            // ReSharper disable once ConditionalAccessQualifierIsNonNullableAccordingToAPIContract
+            return SystemDependencyCollection?.TryResolveType(out entitySystem) ?? false;
         }
 
         /// <inheritdoc />
@@ -383,7 +389,9 @@ public IEnumerable<Type> GetEntitySystemTypes()
 
         public bool TryGetEntitySystem(Type sysType, [NotNullWhen(true)] out object? system)
         {
-            return SystemDependencyCollection.TryResolveType(sysType, out system);
+            system = null;
+            // ReSharper disable once ConditionalAccessQualifierIsNonNullableAccordingToAPIContract
+            return SystemDependencyCollection?.TryResolveType(sysType, out system) ?? false;
         }
 
         public object GetEntitySystem(Type sysType)