Using canvas field to paint on a custom control - delphi

Question

This code generates an error:
Debugger Exception Notification
Project Project3.exe raised exception class $C0000005 with message 'access violation at 0x7757e12c: write of address 0x00000014'.
if Fowner_draw then
begin
canvas.CopyRect(ClientRect, FOD_canvas, ClientRect);
end
I found the solution by deleting pasteBmp.free; line from the code below. It seems like each time copyRect is called the value of FOD_canvas field is assigned again.
procedure Tncrtile.copy_rect(Cimage:timage; source:trect; dest:trect);
var
copyBmp,pasteBmp: TBitmap;
begin
if (Cimage.Picture.Graphic <> nil) and not Cimage.Picture.Graphic.Empty then
begin
copyBmp:=TBitmap.Create;
pasteBmp:=TBitmap.Create;
try
copyBmp.Height:=Cimage.Height;
copyBmp.Width:=Cimage.Width;
pasteBmp.Height:=source.Height;
pasteBmp.Width:=source.Width;
copyBmp.canvas.Draw(0, 0, Cimage.Picture.Graphic);
pasteBmp.Canvas.CopyRect(rect(0, 0, source.Width, source.Height), copyBmp.Canvas, source);
FOD_canvas:=pasteBmp.Canvas;
finally
copyBmp.free;
pasteBmp.free;
end;
Fdrawing_rect:=dest;
Fowner_draw:=true;
invalidate;
end;
end;
Why is this happening? I tried googling and the Delphi help.

Answer 1

As stated in comments, the error is because you are keeping a reference to a destroyed TCanvas and then trying to draw with it. You need to keep a copy of the actual TBitmap instead and then you can draw with it when needed:
constructor Tncrtile.Create(AOwner: TComponent);
begin
inherited;
FOD_Bmp := TBitmap.Create;
end;
destructor Tncrtile.Destroy;
begin
FOD_Bmp.Free;
inherited;
end;
procedure Tncrtile.copy_rect(Cimage: TImage; Source, Dest: TRect);
var
copyBmp, pasteBmp: TBitmap;
begin
if (Cimage.Picture.Graphic <> nil) and (not Cimage.Picture.Graphic.Empty) then
begin
copyBmp := TBitmap.Create;
pasteBmp := TBitmap.Create;
try
copyBmp.Height := Cimage.Height;
copyBmp.Width := Cimage.Width;
pasteBmp.Height := Source.Height;
pasteBmp.Width := Source.Width;
copyBmp.Canvas.Draw(0, 0, Cimage.Picture.Graphic);
pasteBmp.Canvas.CopyRect(Rect(0, 0, Source.Width, Source.Height), copyBmp.Canvas, Source);
FOD_Bmp.Assign(pasteBmp);
finally
copyBmp.Free;
pasteBmp.Free;
end;
Fdrawing_rect := Dest;
Fowner_draw := True;
Invalidate;
end;
end;
...
if Fowner_draw and (not FOD_BMP.Empty) then
begin
Canvas.CopyRect(ClientRect, FOD_Bmp.Canvas, ClientRect);
end