diff --git a/src/common/subsystems.hpp b/src/common/subsystems.hpp
index ea40faad4..103d78d3f 100644
--- a/src/common/subsystems.hpp
+++ b/src/common/subsystems.hpp
@@ -367,17 +367,23 @@ public:
 
     ~Window() {
         //don't delete form_. it is autmatically cleaned up by screen_
-        delete screen_;
-        delete writer_;
-        delete capture_;
-        delete nvgContext_;
-        delete clvaContext_;
-        delete clglContext_;
+        if(screen_)
+            delete screen_;
+        if(writer_)
+            delete writer_;
+        if(capture_)
+            delete capture_;
+        if(nvgContext_)
+            delete nvgContext_;
+        if(clvaContext_)
+            delete clvaContext_;
+        if(clglContext_)
+            delete clglContext_;
         glfwDestroyWindow(getGLFWWindow());
         glfwTerminate();
     }
 
-    cv::Ptr<nanogui::FormHelper> form() {
+    nanogui::FormHelper* form() {
         return form_;
     }
 
@@ -421,6 +427,10 @@ public:
         });
     }
 
+    void makeGLFWContextCurrent() {
+        glfwMakeContextCurrent(getGLFWWindow());
+    }
+
     cv::VideoWriter& makeVAWriter(const string& outputFilename, const int fourcc, const float fps, const cv::Size& frameSize, const int vaDeviceIndex) {
         writer_ = new cv::VideoWriter(outputFilename, cv::CAP_FFMPEG, cv::VideoWriter::fourcc('V', 'P', '9', '0'), fps, frameSize, {
                 cv::VIDEOWRITER_PROP_HW_DEVICE, vaDeviceIndex,
@@ -526,22 +536,22 @@ public:
         offscreen_ = o;
     }
 
-    nanogui::Window& makeWindow(int x, int y, const string& title) {
-        return *form()->add_window(nanogui::Vector2i(x, y), title);
+    nanogui::Window* makeWindow(int x, int y, const string& title) {
+        return form()->add_window(nanogui::Vector2i(x, y), title);
     }
 
-    nanogui::Label& makeGroup(const string& label) {
-        return *form()->add_group(label);
+    nanogui::Label* makeGroup(const string& label) {
+        return form()->add_group(label);
     }
 
-    nanogui::detail::FormWidget<bool>& makeFormVariable(const string &name, bool &v, const string &tooltip = "") {
+    nanogui::detail::FormWidget<bool>* makeFormVariable(const string &name, bool &v, const string &tooltip = "") {
         auto var = form()->add_variable(name, v);
         if (!tooltip.empty())
             var->set_tooltip(tooltip);
-        return *var;
+        return var;
     }
 
-    template<typename T> nanogui::detail::FormWidget<T>& makeFormVariable(const string &name, T &v, const T &min, const T &max, bool spinnable = true, const string &unit = "", const string tooltip = "") {
+    template<typename T> nanogui::detail::FormWidget<T>* makeFormVariable(const string &name, T &v, const T &min, const T &max, bool spinnable = true, const string &unit = "", const string tooltip = "") {
         auto var = form()->add_variable(name, v);
         var->set_spinnable(spinnable);
         var->set_min_value(min);
@@ -550,7 +560,7 @@ public:
             var->set_units(unit);
         if (!tooltip.empty())
             var->set_tooltip(tooltip);
-        return *var;
+        return var;
     }
 
     void setUseOpenCL(bool u) {
diff --git a/src/optflow/optflow-demo.cpp b/src/optflow/optflow-demo.cpp
index bddd31b7b..7168d45d1 100644
--- a/src/optflow/optflow-demo.cpp
+++ b/src/optflow/optflow-demo.cpp
@@ -13,6 +13,7 @@ using std::cerr;
 using std::endl;
 using std::vector;
 using std::string;
+using namespace std::literals::chrono_literals;
 
 /** Application parameters **/
 
@@ -182,7 +183,7 @@ void composite_layers(const cv::UMat background, const cv::UMat foreground, cons
 }
 
 void setup_gui(cv::Ptr<kb::Window> window) {
-    auto subWin = window->makeWindow(6, 45, "Settings");
+    window->makeWindow(6, 45, "Settings");
 
     auto useOpenCL = window->makeFormVariable("Use OpenCL", use_opencl, "Enable or disable OpenCL acceleration");
 
@@ -201,7 +202,7 @@ void setup_gui(cv::Ptr<kb::Window> window) {
     window->makeGroup("Effect");
     window->makeFormVariable("Max. Stroke Size", max_stroke, 1, 100, true, "px", "The theoretical maximum size of the drawing stroke which is scaled by the area of the convex hull of tracked points and therefor is usually much smaller");
     auto glowKernel = window->makeFormVariable("Glow Kernel Size", glow_kernel_size, 1, 63, true, "", "Intensity of glow defined by kernel size");
-    glowKernel.set_callback([](const int& k) {
+    glowKernel->set_callback([](const int& k) {
         glow_kernel_size = std::max(int(k % 2 == 0 ? k + 1 : k), 1);
     });
     auto color = window->form()->add_variable("Color", effect_color);
@@ -243,6 +244,7 @@ int main(int argc, char **argv) {
     }
 
     float fps = capture.get(cv::CAP_PROP_FPS);
+
     window->makeVAWriter(OUTPUT_FILENAME, cv::VideoWriter::fourcc('V', 'P', '9', '0'), fps, window->getSize(), VA_HW_DEVICE_INDEX);
 
     //BGRA