I see that this is in the constructor. The constructor of what? Is it possible that the objects aren't actually available yet?

Could you move this code to another event and do a post() call from the constructor to see if that may be the case?

The height and width of a window has nothing to do with TypeOf()  function. That was logical problem with my code that i fixed later.

But:

TypeOf function do have some problems because it works when window is not descendant neither ancestor.

In my app the parent window has loop on control array in constructor event of a control and right there typeof() fails. Actual reason is still unclear.

There is alternate way to avoid use of TypeOf()

The Error line:

TheParentWin.control[control_counter].Typeof()

I used variables to assign objects in control[] to avoid use of TypeOf() function.

DataWindow TheDW

Tab TheTab

If I don't use  TRY CATCH app will crash

/// when expecting a DW in the control array
TRY
  TheDW = TheParentWin.control[control_counter]
CATCH(RuntimeError RT012)
  SetNull(TheDW)
END TRY

/// when expecting a tab control in the control[] array
TRY
  TheTab = TheParentWin.control[control_counter]
CATCH(RuntimeError RT014)
  SetNull(TheTab)
END TRY

Then i check if the variable hold a valid object by using IsValid()

IF IsValid(TheDW) THEN
  TheDW.ClassNmae()
END IF

That is how i can check how many DWs are there in the  window and how many DWs are in tabpages. All this is done in constructor event.

That means TypeOf() is failing for no reason in my previous code.

There was no need to replace TypeOf() with type casting operator '='

Real problem started when debugger was not showing enumerated datatype that TypeOf() was returning. I thought null was returned every time.

Someone suggested to try same script in PB2019 and this step took me sometime. Good thing is i completed script in PB 2019 and when tested it on PB12.5 it worked nicely without any problem.

just in case someone interested to see what changes were made in script

w_parent TheParentWin
TheParentWin = Parent

uo_tab TheChildTab
uo_dw DW
Integer control_counter
Integer Tabpage, TabPageChild, TabPageOfChildTab

Tab TheTab

/// Main loop on window control[]
FOR control_counter = 1 TO UpperBound(TheParentWin.control)
 	SetNull(DW)
	CHOOSE CASE TheParentWin.control[control_counter].TypeOf()
		CASE DataWindow!
			_errorLog("Window control[] DW :" + TheParentWin.control[control_counter].ClassName() )
			AllDW[UpperBound(AllDW) + 1] = TheParentWin.control[control_counter]
			
			/// Win is instance of w_parent (refer to parent win). uo_dw has another 
			// instance var Win of type w_parent. InView is boolean var of uo_dw.
			/// Either crashing on DW.Win or on DW.InView means that the
			/// datawindow control used in a descendant window is not a uo_dw.
			TRY
				DW = TheParentWin.control[control_counter]
				DW.Win = TheParentWin
				IF DW.X < TheParentWin.Width AND DW.Y < TheParentWin.Height THEN
					DW.InView = TRUE
				ELSE
					/// outsider
					DW.InView = FALSE
				END IF
			CATCH (RuntimeError RTE0011)
				_errorLog("Simple dw <> uo_dw: " + DW.ClassName() + " In window: " + TheParentWin.ClassName())
			END TRY
			/// go for next in control[]
			Continue
		CASE Tab!
			
	END CHOOSE
	
	/// =============== TAB control[] =========================
	/// Object if not a DW, could be a Tab control
	CHOOSE CASE TheParentWin.control[control_counter].TypeOf()
		CASE Tab!
			TheTab = TheParentWin.control[control_counter]
			_errorLog("Wondow control[] Tab:" + TheParentWin.control[control_counter].ClassName() )

			/// How many tabpages?
			FOR Tabpage = 1 TO UpperBound(TheTab.control)
				/// going inside a tabpage
				/// how many controls on each tabpage?
				FOR TabPageChild = 1 TO UpperBound(TheTab.control[Tabpage].control)
					CHOOSE CASE TheTab.control[Tabpage].control[TabPageChild].TypeOf()
					CASE DataWindow!			
						_errorLog("Valid TABDW :" + TheTab.control[Tabpage].control[TabPageChild].ClassName()) 
							AllDW[UpperBound(AllDW) + 1] = TheTab.control[Tabpage].control[TabPageChild]
							TRY
								DW = TheTab.control[Tabpage].control[TabPageChild]
								DW.Win = TheParentWin
								IF DW.X < TheParentWin.Width AND DW.Y < TheParentWin.Height THEN
									DW.InView = TRUE
								ELSE
									/// outsider
									DW.InView = FALSE
								END IF
							CATCH (RuntimeError RTE0111)
								_errorLog("Simple dw <> uo_dw: " + DW.ClassName() + " In Tab: " + TheTab.control[Tabpage].ClassName())
							END TRY					
					END CHOOSE
				NEXT /// TABPage
			NEXT ///TAB
	END CHOOSE
NEXT