Window looses its handle when docking / undocking

Question

I am using a PageControl on my main form to serve as a dock site for other forms.

One of these forms, fPanelSonar, has a panel that serves as container for a 3rd party application that I load as follow:

var
  i : Integer;
  ExecutePingViewer : string;
  sePingViewer,seOVREC: TShellExecuteInfo;
begin
  Timer1.Enabled := False;
  fPanelSonar.hdlPing360 := FindWindow(nil, PChar('Ping Viewer - v2.0.4'));
  if fPanelSonar.hdlPing360 = 0 then
  begin
    ExecutePingViewer:='C:\Program Files (x86)\Oceanvault\pingviewer_release\deploy\pingviewer.exe';
    FillChar(sePingViewer, SizeOf(sePingViewer), 0) ;
    sePingViewer.cbSize := SizeOf(TShellExecuteInfo) ;
    with sePingViewer do
    begin
      fMask := SEE_MASK_NOCLOSEPROCESS;
      Wnd := 0;
      lpFile := PChar(ExecutePingViewer) ;
      nShow := SW_HIDE;
    end;
    if ShellExecuteEx(@sePingViewer) then
    begin
      fPanelSonar.hdlPing360 := FindWindow(nil, PChar('Ping Viewer - v2.0.4'));
      Windows.SetParent(hdlPing360, pSonar.Handle);
      ShowWindow(hdlPing360, SW_SHOWMAXIMIZED);
    end
    else ShowMessage('Error loading Ping360!') ;
  end;

This works well and my 3rd party application is loaded as I want.

The problem is when I drop fPanelSonar into the PageControl, the 3rd party application disappears even if its process "pingviewer.exe" is still running.

Since I saved the hwnd fPanelSonar.hdlPing360 I should be able to set it back to its parent again, but it appears although this handle is invalid because it doesn't work. (Even if I undock it).

Furthermore, I can't find any other handle with once I dock/undock:

FindWindowEx(fPanelSonar.pSonar.Handle,0,nil, PChar('Ping Viewer - v2.0.4'));

I can still find the process ID with this function I found off the forum so I know its not killed. (I can also see it in window's Task Manager)

function GetPIDbyProcessName(processName:String):integer;
var
  GotProcess: Boolean;
  tempHandle: tHandle;
  procE: tProcessEntry32;
begin
  tempHandle:=CreateToolHelp32SnapShot(TH32CS_SNAPALL, 0);
  procE.dwSize:=SizeOf(procE);
  GotProcess:=Process32First(tempHandle, procE);
  {$B-}
    if GotProcess and (procE.szExeFile <> processName) then
      repeat GotProcess := Process32Next(tempHandle, procE);
      until (not GotProcess) or (procE.szExeFile = processName);
  {$B+}

  if GotProcess then
    result := procE.th32ProcessID
  else
    result := 0; // process not found in running process list

  CloseHandle(tempHandle);
end;

I tried to find the hwd with process ID but it doens't work.

Note that if I dock fPanelSonar before I assign the pingviewer.exe parent, It does work.

At present, I can use the dock/undock events to kill the pingviewer process and load a new one but that is awfully messy...

So I am a bit confused why this is happening and how to solve it properly.

Cheers, E.