You can test it locally if you use Chrome DevTools. When you open DevTools
you can click on the small mobile device icon on the left side right next
to the search icon. Once you have done that you should see a new dark
toolbar at the top of your page which has a Network drop down which allows
you to emulate slower network conditions in Chrome.
Given your stack trace it seems like you called
AbsolutePanel.add(childWidget) somewhere but your instance of AbsolutePanel
was null.
For your understanding how to read these stack frames:
__gwt$exception: <skipped>: Cannot read property
'com_google_gwt_user_client_ui_UIObject_element' of null
at
Unknown.com_google_gwt_user_client_ui_AbsolutePanel_$add__Lcom_google_gwt_user_client_ui_AbsolutePanel_2Lcom_google_gwt_user_client_ui_Widget_2V
First line is the JavaScript exception message which says you have tried to
access UIObject.element but UIObject was null. Since UIObject has a
getElement() method that means your java code did call "null.getElement()".
At the next line you see that this happened in
AbsolutePanel.$add(AbsolutePanel, Widget). This method has been generated
by the GWT compiler and is a static version of AbsolutePanel.add(Widget).
These generated static methods are marked with "$" by GWT. Given the
implementation of AbsolutePanel.add(Widget) it seems like your instance of
AbsolutePanel was null.
Your code did something like:
AbsolutePanel myPanel = null; // somehow the variable was null
myPanel.add(child);
with AbsolutePanel.add() being implemented as:
public void add(Widget w) {
super.add(w, *getElement()*);
}
GWT compiler changed your method call to a static method call
AbsolutePanel.$add(myPanel, w);
And the implementation of $add is probably similar to
public static void $add(AbsolutePanel instance, Widget w) {
ComplexPanel.$add(w, *instance.getElement()*);
}
The bold code causes the exception. Since getElement() and the element
field are defined on UiObject the exception message says you have tried to
access UiObject.element with UiObject being null.
-- J.
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