Added TCEFSentinel to more demos

- Check that all frames are valid before using them
- Added an error page to MiniBrowser.

demos/Delphi_VCL/SimpleOSRBrowser/uSimpleOSRBrowser.dfm

@@ -160,4 +160,9 @@ object Form1: TForm1
     Left = 24
     Top = 206
   end
+  object CEFSentinel1: TCEFSentinel
+    OnClose = CEFSentinel1Close
+    Left = 24
+    Top = 350
+  end
 end

demos/Delphi_VCL/SimpleOSRBrowser/uSimpleOSRBrowser.pas

@@ -50,7 +50,8 @@ uses
   Windows, Messages, SysUtils, Variants, Classes, SyncObjs,
   Graphics, Controls, Forms, Dialogs, StdCtrls, ExtCtrls, AppEvnts,
   {$ENDIF}
-  uCEFChromium, uCEFTypes, uCEFInterfaces, uCEFConstants, uCEFBufferPanel;
+  uCEFChromium, uCEFTypes, uCEFInterfaces, uCEFConstants, uCEFBufferPanel,
+  uCEFSentinel;
 
 const
   // Set this constant to True and load "file://transparency.html" to test a
@@ -69,6 +70,7 @@ type
     SaveDialog1: TSaveDialog;
     Timer1: TTimer;
     Panel1: TBufferPanel;
+    CEFSentinel1: TCEFSentinel;
 
     procedure AppEventsMessage(var Msg: tagMSG; var Handled: Boolean);
 
@@ -113,6 +115,7 @@ type
 
     procedure Timer1Timer(Sender: TObject);
     procedure ComboBox1Enter(Sender: TObject);
+    procedure CEFSentinel1Close(Sender: TObject);
 
   protected
     FPopUpBitmap     : TBitmap;
@@ -174,7 +177,8 @@ uses
 // 2- chrmosr.CloseBrowser(True) will trigger chrmosr.OnClose and we have to
 //    set "Result" to false and CEF3 will destroy the internal browser immediately.
 // 3- chrmosr.OnBeforeClose is triggered because the internal browser was destroyed.
-//    Now we set FCanClose to True and send WM_CLOSE to the form.
+//    Now we call TCEFSentinel.Start, which will trigger TCEFSentinel.OnClose when the renderer processes are closed.
+// 4- TCEFSentinel.OnClose sets FCanClose := True and sends WM_CLOSE to the form.
 
 procedure CreateGlobalCEFApp;
 begin
@@ -326,8 +330,7 @@ end;
 
 procedure TForm1.chrmosrBeforeClose(Sender: TObject; const browser: ICefBrowser);
 begin
-  FCanClose := True;
-  PostMessage(Handle, WM_CLOSE, 0, 0);
+  CEFSentinel1.Start;
 end;
 
 procedure TForm1.chrmosrBeforePopup(Sender : TObject;
@@ -894,6 +897,12 @@ begin
             (cardinal(aCurrentTime - FLastClickTime) > GetDoubleClickTime);
 end;
 
+procedure TForm1.CEFSentinel1Close(Sender: TObject);
+begin
+  FCanClose := True;
+  PostMessage(Handle, WM_CLOSE, 0, 0);
+end;
+
 procedure TForm1.Panel1Enter(Sender: TObject);
 begin
   chrmosr.SendFocusEvent(True);