Fixed warnings around nullable reference types

Author: fubar-coder

FubarDev.FtpServer.ruleset

@@ -2,6 +2,8 @@
 <RuleSet Name="Regeln für FubarDev.FtpServer" Description="Codeanalyseregeln für FubarDev.FtpServer.csproj." ToolsVersion="14.0">
   <Rules AnalyzerId="StyleCop.Analyzers" RuleNamespace="StyleCop.Analyzers">
     <Rule Id="SA0001" Action="None" />
+    <!-- StyleCop.Analyzers cannot handle C# 8.0 yet -->
+    <Rule Id="SA1009" Action="None" />
     <Rule Id="SA1101" Action="None" />
     <Rule Id="SA1200" Action="None" />
     <Rule Id="SA1309" Action="None" />

src/FubarDev.FtpServer/FtpConnection.cs

@@ -463,7 +463,7 @@ await _serverCommandExecutor.ExecuteAsync(response, cancellationToken)
             catch (Exception ex)
             {
                 var exception = ex;
-                while (exception is AggregateException aggregateException)
+                while (exception is AggregateException aggregateException && aggregateException.InnerException != null)
                 {
                     exception = aggregateException.InnerException;
                 }

src/FubarDev.FtpServer/FtpServer.cs

@@ -188,7 +188,7 @@ await AddClientAsync(client)
             catch (Exception ex)
             {
                 var exception = ex;
-                while (exception is AggregateException aggregateException)
+                while (exception is AggregateException aggregateException && aggregateException.InnerException != null)
                 {
                     exception = aggregateException.InnerException;
                 }
@@ -292,9 +292,9 @@ await connection.StartAsync()
             }
         }
 
-        private void ConnectionOnClosed(object sender, EventArgs eventArgs)
+        private void ConnectionOnClosed(object? sender, EventArgs eventArgs)
         {
-            var connection = (IFtpConnection)sender;
+            var connection = (IFtpConnection)(sender ?? throw new InvalidOperationException("Missing sender information."));
             if (!_connections.TryRemove(connection, out var info))
             {
                 return;

src/FubarDev.FtpServer/ServerCommands/DelegateServerCommandExecutor.cs

@@ -51,7 +51,7 @@ public Task ExecuteAsync(IServerCommand serverCommand, CancellationToken cancell
                 _serverCommandHandlerDelegates.Add(serverCommandType, cmdDelegate);
             }
 
-            return (Task)cmdDelegate.DynamicInvoke(serverCommand, cancellationToken);
+            return (Task)cmdDelegate.DynamicInvoke(serverCommand, cancellationToken)!;
         }
     }
 }

src/FubarDev.FtpServer/ServerCommands/ReflectionServerCommandExecutor.cs

@@ -40,13 +40,13 @@ public Task ExecuteAsync(IServerCommand serverCommand, CancellationToken cancell
                 var executeAsyncMethod = handlerType.GetRuntimeMethod("ExecuteAsync", new[] { serverCommandType, typeof(CancellationToken) });
                 var handler = _ftpConnectionAccessor.FtpConnection.ConnectionServices.GetRequiredService(handlerType);
 
-                commandHandlerInfo = new CommandHandlerInfo(handler, executeAsyncMethod);
+                commandHandlerInfo = new CommandHandlerInfo(handler, executeAsyncMethod!);
                 _serverCommandHandlerInfo.Add(serverCommandType, commandHandlerInfo);
             }
 
             return (Task)commandHandlerInfo.ExecuteMethodInfo.Invoke(
                 commandHandlerInfo.CommandHandler,
-                new object[] { serverCommand, cancellationToken });
+                new object[] { serverCommand, cancellationToken })!;
         }
 
         private class CommandHandlerInfo