Make sure to select the right IAnalyzerResult from Buildalyzer

Fixes #1899

src/Stryker.Core/Stryker.Core/Initialisation/ProjectFileReader.cs

@@ -104,7 +104,7 @@ private IAnalyzerResult SelectAnalyzerResult(IAnalyzerResults analyzerResults, s
         {
             if (targetFramework == null)
             {
-                return analyzerResults.First();
+                return analyzerResults.First(e => e.TargetFramework != null);
             }
 
             var analyzerResultForFramework = analyzerResults.SingleOrDefault(result => result.TargetFramework == targetFramework);