GUI Layout flickering while exiting application

Questions & Answers
1

When user press back button for exit, I am showing a dialog made of GUI components. When i press yes button for exit the layout flickers for sometime and then the application exit. Here is the script .

public class MainScreenGUIScript : MonoBehaviour 
{
	public bool visible;
	public GUISkin customSkin;

	float screenWidth;
	float screenHeight;

 	float dialogStartX;
 	float dialogStartY;
 	float dialogWidth;
 	float dialogHeight;

 	float DialoglabelX;
 	float DialoglabelY;

 	float DialogTitlelabelX;
 	float DialogTitlelabelY;

 	float DialoglabelWidth;
 	float DialoglabelHeight;

 	float DialogYesButtonX;
 	float DialogYesButtonY;

 	float DialogNoButtonX;
 	float DialogNoButtonY;

 	float DialogButtonWidth;
 	float DialogButtonHeight;

	public GUIStyle boxBackgroundStyle;
	public GUIStyle yesButtonStyle;
	public GUIStyle noButtonStyle;
	public GUIStyle titleLabelStyle;
	public GUIStyle fbStyle;
	public GUIStyle settingStyle;

	float buttonWidth;
 	float buttonHeight;
 	float buttonY;
 	float othersX;
 	float backX;

	void Start () 
	{
		visible=false;
	}
	
	// Update is called once per frame
	void OnGUI()
	{
		GUI.skin = customSkin;

		screenWidth=Screen.width;
		screenHeight=Screen.height;

		dialogStartX=(float) (screenWidth*0.275);
		dialogStartY=(float) (screenHeight*0.35);

		dialogWidth=(float) (screenWidth*0.5);
		dialogHeight=(float) (screenHeight*0.28);

		DialoglabelWidth=(float) (screenWidth*0.3);
		DialoglabelHeight=(float) (screenHeight*0.1);

		DialoglabelX=(float) (screenWidth*0.415);
		DialoglabelY=(float) (screenHeight*0.45);

		DialogTitlelabelX=(float) (screenWidth*0.4);
		DialogTitlelabelY=(float) (screenHeight*0.36);

		DialogYesButtonX=(float) (screenWidth*0.36);
		DialogYesButtonY=(float) (screenHeight*0.54);

		DialogNoButtonX=(float) (screenWidth*0.55);
		DialogNoButtonY=(float) (screenHeight*0.54);

		DialogButtonWidth=(float) (screenWidth*0.15);
		DialogButtonHeight=(float) (screenHeight*0.08);

		buttonWidth=(float) (screenWidth*0.06);
		buttonHeight=(float) (screenHeight*0.08);

		othersX=(float) (screenWidth*0.9);
		backX=(float) (screenWidth*0.01);
		buttonY=(float) (screenHeight*0.89);

		if(GUI.Button(new Rect(othersX,buttonY,buttonWidth,buttonHeight),"",fbStyle))
		{
			if(!visible)
			{
				Application.OpenURL ("");
			}	
		}
		if(GUI.Button(new Rect(backX,buttonY,buttonWidth,buttonHeight),"",settingStyle))
		{
    		if(!visible)
			{
				Application.LoadLevel(3);	
			}	
		}
		if(visible)
		{
			GUI.Box(new Rect(0,0,Screen.width,Screen.height),"");
			GUI.Box(new Rect(dialogStartX,dialogStartY,dialogWidth,dialogHeight),"",boxBackgroundStyle);
			GUI.Label(new Rect(DialogTitlelabelX,DialogTitlelabelY,DialoglabelWidth,DialoglabelHeight),"Quit",titleLabelStyle);
			GUI.Label(new Rect(DialoglabelX,DialoglabelY,DialoglabelWidth,DialoglabelHeight),"Do You Want To Quit ?");
			if(GUI.Button(new Rect(DialogYesButtonX,DialogYesButtonY,DialogButtonWidth,DialogButtonHeight),"",yesButtonStyle))
			{				
				visible=false;
				applicationExit();
			}
			if(GUI.Button(new Rect(DialogNoButtonX,DialogNoButtonY,DialogButtonWidth,DialogButtonHeight),"",noButtonStyle))
			{
				visible=false;
			}
		}
		if (Input.GetKey(KeyCode.Escape))
 	  	{
			visible=true;
    	}	
	}

	void applicationExit()
	{
		Application.Quit();
	}
}

Initially there are two GUI.Button which are visible. When user press back key(Android) I change the value of visible to true so that dialog appears . When user clicks on Yes Button I set value of visible to false and then call Application.Quit() it flickers and then application exit. What could be the reason.

2

When upgraded to unity 4.3.1 it is working fine. No flickering & nothing so it might be something with the unity not sure though.