DockLayoutPanel.getOffsetHeight return zero even after calling forceLayout()

there is a simplePanel and the simplePanel add a DockLayoutPanel called AdvancedSearchPanel.
in the AdvancedSearchPanel constructor, the width is set to 100% and height to 1800px.

then there is a menu item and when it is clicked,
I try to add the AdvancedSearchLogsPanel() to simplePanel and then called
advancedSearchLogsPanel.forceLayout()
before calling
advancedSearchLogsPanel.getOffsetWidth() and advancedSearchLogsPanel.getOffsetHeight()
but both return zero!!

what is the problem come from?

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