Fixed a crash when generating any C# bindings on Windows.

It was a regression from the dropping of the C# option for a single generated source file.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

src/Generator/Generator.cs

@@ -118,7 +118,8 @@ namespace CppSharp.Generators
                         FilePath = string.Format("{0}.cs", module.LibraryName),
                         Module = module
                     },
-                    Templates = Generate(module.Units)
+                    Templates = Generate(module.Units.Where(
+                        u => u.IsGenerated && u.HasDeclarations && !u.IsSystemHeader && u.IsValid))
                 };
                 output.Templates[0].Process();
                 outputs.Add(output);

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -198,10 +198,11 @@ namespace CppSharp.Generators.CSharp
             }
             PopBlock(NewLineKind.BeforeNextBlock);
 
-            if (!string.IsNullOrEmpty(TranslationUnit.Module.OutputNamespace))
+            var module = TranslationUnits.Count == 0 ? Module.SystemModule : TranslationUnit.Module;
+            if (!string.IsNullOrEmpty(module.OutputNamespace))
             {
                 PushBlock(CSharpBlockKind.Namespace);
-                WriteLine("namespace {0}", TranslationUnit.Module.OutputNamespace);
+                WriteLine("namespace {0}", module.OutputNamespace);
                 WriteStartBraceIndent();
             }
 
@@ -210,7 +211,7 @@ namespace CppSharp.Generators.CSharp
                 GenerateDeclContext(unit);
             }
 
-            if (!string.IsNullOrEmpty(TranslationUnit.Module.OutputNamespace))
+            if (!string.IsNullOrEmpty(module.OutputNamespace))
             {
                 WriteCloseBraceIndent();
                 PopBlock(NewLineKind.BeforeNextBlock);