Fix possible null reference exceptions.

src/Avalonia.Controls/TopLevel.cs

@@ -179,7 +179,7 @@ namespace Avalonia.Controls
         double ILayoutRoot.LayoutScaling => PlatformImpl?.Scaling ?? 1;
 
         /// <inheritdoc/>
-        double IRenderRoot.RenderScaling => PlatformImpl.Scaling;
+        double IRenderRoot.RenderScaling => PlatformImpl?.Scaling ?? 1;
 
         IStyleHost IStyleHost.StylingParent
         {

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -327,7 +327,7 @@ namespace Avalonia.Rendering
             catch (RenderTargetCorruptedException ex)
             {
                 Logging.Logger.Information("Renderer", this, "Render target was corrupted. Exception: {0}", ex);
-                _renderTarget.Dispose();
+                _renderTarget?.Dispose();
                 _renderTarget = null;
             }
         }

src/Avalonia.Visuals/Rendering/SceneGraph/BrushDrawOperation.cs

@@ -46,7 +46,7 @@ namespace Avalonia.Rendering.SceneGraph
         protected Pen ToImmutable(Pen pen)
         {
             var brush = pen?.Brush != null ? ToImmutable(pen.Brush) : null;
-            return ReferenceEquals(pen?.Brush, brush) ?
+            return pen == null || ReferenceEquals(pen?.Brush, brush) ?
                 pen :
                 new Pen(
                     brush,