fix: Linux Framebuffer nullable

src/Linux/Avalonia.LinuxFramebuffer/LinuxFramebufferPlatform.cs

@@ -154,7 +154,7 @@ public static class LinuxFramebufferPlatformExtensions
         var lifetime = LinuxFramebufferPlatform.Initialize(builder, outputBackend, inputBackend);
         builder.SetupWithLifetime(lifetime);
         lifetime.Start(args);
-        builder.Instance.Run(lifetime.Token);
+        builder.Instance!.Run(lifetime.Token);
         return lifetime.ExitCode;
     }
 }

src/Linux/Avalonia.LinuxFramebuffer/Output/DrmOutput.cs

@@ -43,13 +43,13 @@ namespace Avalonia.LinuxFramebuffer.Output
         public IPlatformGraphics PlatformGraphics { get; private set; }
 
         public DrmOutput(DrmCard card, DrmResources resources, DrmConnector connector, DrmModeInfo modeInfo,
-            DrmOutputOptions? options = null)
+            DrmOutputOptions options = null)
         {
             if(options != null) 
                 _outputOptions = options;
             Init(card, resources, connector, modeInfo);
         }
-        public DrmOutput(string path = null, bool connectorsForceProbe = false, DrmOutputOptions? options = null)
+        public DrmOutput(string path = null, bool connectorsForceProbe = false, DrmOutputOptions options = null)
         {
             if(options != null) 
                 _outputOptions = options;
@@ -63,7 +63,7 @@ namespace Avalonia.LinuxFramebuffer.Output
             if(connector == null)
                 throw new InvalidOperationException("Unable to find connected DRM connector");
 
-            DrmModeInfo? mode = null;
+            DrmModeInfo mode = null;
 
             if (options?.VideoMode != null)
             {