Fast fix for NullRefException from FileAppender

Author: erikmav

src/log4net.Tests/Appender/RollingFileAppenderTest.cs

@@ -2013,6 +2013,17 @@ public void TestInitializeRollBackups4()
       VerifyInitializeRollBackups(8, iMaxRollBackups);
     }
 
+    /// <summary>
+    /// Ensures that no problems result from creating and then closing the appender
+    /// when it has not also been initialized with ActivateOptions().
+    /// </summary>
+    [Test]
+    public void TestCreateCloseNoActivateOptions()
+    {
+      var appender = new RollingFileAppender();
+      appender.Close();
+    }
+
     /// <summary>
     /// 
     /// </summary>

src/log4net.Tests/log4net.Tests.csproj

@@ -68,6 +68,7 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
     <PackageReference Include="NUnit" Version="3.14.0" />
     <PackageReference Include="NExpect" Version="1.0.273" />
     <PackageReference Include="Quackers.TestLogger" Version="1.0.24" />

src/log4net/Appender/FileAppender.cs

@@ -169,7 +169,7 @@ private LockStateException(SerializationInfo info, StreamingContext context) : b
       }
 
       private Stream m_realStream = null;
-      private LockingModelBase m_lockingModel = null;
+      private readonly LockingModelBase m_lockingModel;
       private int m_lockLevel = 0;
 
       public LockingStream(LockingModelBase locking)
@@ -1306,7 +1306,7 @@ protected override void Reset()
     protected override void OnClose()
     {
       base.OnClose();
-      m_lockingModel.OnClose();
+      m_lockingModel?.OnClose();
     }
 
     /// <summary>