Access violation in Lazarus - pointers

Question

I've been doing my university project and I've stumbled upon problem I cannot solve.
After declaring variable (a pointer to TButton) I cannot use it. I'll copy a part of my code to help show the problem.
So before the implementation I have
private
prevButton : ^TButton;
After that I use OnClick procedure with my buttons
procedure TForm2.MissingButtonClick(Sender : TObject);
var
b : TButton;
begin
b := Sender as TButton;
prevButton := #b;
showmessage(prevButton^.Caption);
end;
And caption is shown no problem. But then when I use my OnClick procedure and try to change labels caption I get access violation.
procedure TForm2.LabelClick(Sender : TObject);
var
l : TLabel;
begin
l := Sender as TLabel;
if prevButton = nil then
showmessage('nil');
if prevButton <> nil then begin
showmessage(prevButton^.Caption);
l.Caption := (prevButton^.Caption);
prevButton^.OnClick := #AlreadyClicked;
prevButton^.Free;
prevButton^ := nil;
prevButton := nil;
refreshLabels(words);
end;
end;
So here is the question, why can't I use my variable in this procedure if I could use it without problem second earlier in other procedure.
Cheers.

Answer 1

Your code is severely flawed in several places.
First, the initial declaration is incorrect. A variable of type TButton is already a pointer, so you don't have to dereference it:
private
prevButton: TButton;
You also don't have to dereference it when assigning to it or using it:
procedure TForm2.MissingButtonClick(Sender : TObject);
begin
prevButton := TButton(Sender);
showmessage(prevButton.Caption);
end;
I'm not sure what your LabelClick event is trying to accomplish. If you free prevButton, you also invalidate the original button to which you pointed it. In other words, if you assigned prevButton := Button1;, and then prevButton.Free;, you've also freed Button1, and I don't think that's your intent. You can safely assign nil to prevButton, but don't free it also:
prevButton := nil; // No longer points to existing object instance
If you want to change the OnClick of an existing button, don't jump through all of those hoops:
// Assign this to the button's OnClick in the Object Inspector
procedure TForm1.Button1FirstClick(Sender: TObject);
begin
ShowMessage('First time I was clicked');
Button1.OnClick := #Button1AfterClicked;
end;
// Create these two in the code editor, and declare it in the
// form declaration, but don't assign it to an event of the button
// in the Object Inspector
procedure TForm1.Button1AfterClick(Sender: TObject);
begin
ShowMessage('Second time clicked');
Button1.OnClick := #Button1MoreClicks;
end;
procedure TForm1.Button1MoreClicks(Sender: TObject);
begin
ShowMessage('Third and later clicks');
end;