Where are you calling the 'call export;' statement? If it is during reload the screen objects are not at that point rendered so can not be accessed.
I would suggest that you test your macro by assigning a call to it as an action on a button. If this works you could look at triggering on Post Reload.
Please see my response on this other thread for more details:
Please create the trigger for calling the macro on post loading.
Settings-->Doc properties--:Add -->Run Macro-->Provide the Macro name
In Edit Module write the below macro code.
set obj1 = ActiveDocument.GetSheetObject("CH01")