Sanitize some OpenGL rendering classes

Rendering/OpenGL2/vtkOpenGLFXAAFilter.cxx

@@ -324,6 +324,12 @@ void vtkOpenGLFXAAFilter::ApplyFilter()
   }
 
   vtkShaderProgram* program = this->QHelper->Program;
+  if (!program)
+  {
+    vtkWarningMacro(
+      "Unable to retrieve shader program from internal vtkOpenGLQuadHelper instance.");
+    return;
+  }
   program->SetUniformi("Input", this->Input->GetTextureUnit());
   float invTexSize[2] = { 1.f / static_cast<float>(this->Viewport[2]),
     1.f / static_cast<float>(this->Viewport[3]) };

Rendering/OpenGL2/vtkOpenGLQuadHelper.cxx

@@ -96,6 +96,10 @@ void vtkOpenGLQuadHelper::ReleaseGraphicsResources(vtkWindow*)
   {
     this->VAO->ReleaseGraphicsResources();
   }
+
+  // Owner is shader cache. When the render window releases it's graphic ressources,
+  // OpenGL state is deleted, so the cache is deleted as well.
+  this->Program = nullptr;
 }
 
 //------------------------------------------------------------------------------