How do I enable SSL in TestComplete

I would avoid using the Exists method on the window object itself in this case.  The issue is you'll take a performance hit of several seconds when the window no longer exists.  That's because "Exists" waits for the full amount of time specified in your Auto-wait timeout set in the project properties. You could reset the Auto-wait time per Hassan_Ballan's suggestion.

However, I would suggest using WaitWindow with a loop something like below.  As you can see, I included a counter/timer so that it doesn't get stuck in the loop if the window never closes.

Set myWindow = xxx.WaitWindow(wndClass, wndCaption, 1, 500)
  x = 0
  While myWindow.Exists and x < 10
    Delay 500
    Set myWindow = xxx.WaitWindow(wndClass, wndCaption, 1, 500)
    x = x +1
  Wend
  
  If Not myWindow.Exists Then
    Log.CheckPoint "Yippie!  The window closed"
    Else
    Log.Message "Sad face.  After waiting five seconds the window is still open"
  End If

Yes you can use the property exists described in https://support.smartbear.com/testcomplete/docs/reference/test-objects/members/common-for-all/exists-property.html?sbsearch=exists 

Also as it would wait for the system default value when it no longer exist you need to change the value to lower setting, the following example is in JavaScript.

function Test()
{
  //Capture default timeout value
  var TimeoutValue;
  TimeoutValue = Options.Run.Timeout;
  
  //Set new default timeout value
  Options.Run.Timeout = 500;
  while(Aliases.browser.page.form.panel.Exists == true)
  {
    Delay(500);
  }
  Log.Message("Object no longer exists", "");

  //Restore default timeout value
  Options.Run.Timeout = TimeoutValue;
}

You could use a while loop, This one is writen in JavaScript but should be about the same:

I added a check that after a few checks it returns