Remove seg faults (#87)

* Removed double OpenGl3Shutdown, leading to a segmentation fault in Main.hs.
* Changed nullPtr passing with Maybe to use DearImGui default arguments.

Main.hs

@@ -38,8 +38,6 @@ main = do
       tab2 <- newIORef True
       loop w checked color slider r pos size' selected tab1 tab2
 
-      openGL3Shutdown
-
 
 loop
   :: Window