Add additional null checks in JSONConfiguration

Author: Deadpikle

src/NetSparkle/Configurations/JSONConfiguration.cs

@@ -110,10 +110,10 @@ public virtual string GetSavePath()
             else
             {
 
-                if (string.IsNullOrEmpty(AssemblyAccessor.AssemblyCompany) || string.IsNullOrEmpty(AssemblyAccessor.AssemblyProduct))
+                if (string.IsNullOrEmpty(AssemblyAccessor?.AssemblyCompany) || string.IsNullOrEmpty(AssemblyAccessor?.AssemblyProduct))
                 {
                     throw new NetSparkleException("Error: NetSparkleUpdater is missing the company or product name tag in the assembly accessor ("
-                        + AssemblyAccessor.GetType() + ")");
+                        + (AssemblyAccessor?.GetType().ToString() ?? "[null]") + ")");
                 }
                 var applicationFolder = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData, Environment.SpecialFolderOption.DoNotVerify);
                 var saveFolder = Path.Combine(applicationFolder, AssemblyAccessor.AssemblyCompany, AssemblyAccessor.AssemblyProduct, "NetSparkleUpdater");