use First instead of FirstOrDefault

Author: tomjebo

samples/Linq/SvgExample/StronglyTypedTools.cs

@@ -28,12 +28,12 @@ public static void AddSvg(Stream stream, string svgPath, double percentageOfCy)
             }
 
             // Get relationship ID of first slide.
-            string sldRelId = presentationPart
+            string? sldRelId = presentationPart
                 .Presentation
                 .SlideIdList?
                 .Elements<Presentation.SlideId>()
                 .Select(slideId => (string)slideId.RelationshipId!)
-                .FirstOrDefault() ?? throw new InvalidOperationException("Presentation has no slides.");
+                .First();
 
             // Get first slide's part.
             var slidePart = (SlidePart)presentationPart.GetPartById(sldRelId);