Hold GUI event thread until until method finish

Question

Currently in my application the user is able to create windows (no controls on the as of yet) which are docked to a pagecontrol. Each editor has a set of toolboxes associated with them. This set is reset upon editor construction and modified as toolboxes are closed and opened. When the page control changes tab the state of the toolboxes is saved, they are closed and then the toolboxes for the new tab are restored.

My application is crashing (seemingly randomly at first) when you change tab. I believe it is when you change tab very fast (the speed of a double click) the processing of the toolboxes in the PageControl.OnChange event doesnt complete before the next PageControl.OnChange event starts and I end up with two events running in 'parallel'. My questions are:

1) Does this sound like a plausible theory? I am aware there is a GUI processing thread does delphi work by dispatching new threads to deal with events?

2) How would you suggest going about fixing this? Hold the messages until the first method finishes (it is a very quick method so there shouldnt be any use lag really)?

Here is some code incase anyone would like to look at the problem from another angle.

This is the code for the onchange event.

procedure TMainForm.PageDockingAreaChange(Sender: TObject);
var
  count : integer;

  // Shortcut variables
  FormShortcut : TForm;
  WelcomeShortcut : TWelcomePageForm;
  BaseEditorShortcut : TBaseEditor;

begin

  // Set nil values
  FormShortcut := nil;
  WelcomeShortcut := nil;
  BaseEditorShortcut := nil;

  // Create shortcut value
  FormShortcut := Self.GetActiveForm;
  if (FormShortcut is TWelcomePageForm) then WelcomeShortcut := TWelcomePageForm(FormShortcut);
  if (FormShortcut is TBaseEditor) then BaseEditorShortcut := TBaseEditor(FormShortcut);

  // Hide all tabs when welcome page is visible
  if (WelcomeShortcut <> nil) then
    begin

      // Clear any existing toolboxes
      Self.ToolboxClearAll;

    end;
  {endif}

  // Try to execute toolbox setup
  try

    // Load toolbox state
    Self.ToolboxSetup(BaseEditorShortcut);

  except

    // Display fatal error to the user
    ShowMessage('Fatal Error : Unable to load toolboxes.');

  end;

  // Update last active editor
  if (BaseEditorShortcut = nil) then
    Self.LastActiveEditor := nil
  else
    Self.LastActiveEditor := BaseEditorShortcut;
  {endif}

end;

This method closes all toolboxes and frees them in turn.

procedure TMainForm.ToolboxClearAll;
var
  count : integer;
begin

  // Save toolbox state if needed
  if Assigned(Self.LastActiveEditor) then
    Self.LastActiveEditor.SaveToolboxState;

  // Close and free all toolboxes
  for count := 0 to length(Self.ActiveToolboxes)-1 do
    Self.ToolboxCloseAndFree(Self.ActiveToolboxes[count]);

  // Reset array size (should already be done though)
  SetLength(Self.ActiveToolboxes, 0);

end;