Popup error on Linux

The error tells you exactly what is wrong:
"Access is allowed from event dispatch thread only."

Yes, but I don't know what it means, and I'm confused because in Windows it's working fine.
Please help me to solve or understand this problem Dimitry, I don't know what I'm doing wrong.

The problem is actually independent of the platform. The getGotoDeclarationTargets() method is not only used to perform the actual navigation, but also for other purposes (in this case, it's called from the code that is calculating the contents of the Ctrl-hover popup). Because of that, it's not appropriate to show any UI in an implementation of this method.

OK, but how can I solve my problem then?
Because the GotoDeclarationAction has this code:

public static boolean chooseAmbiguousTarget(final Editor editor,
                                              int offset,
                                              PsiElementProcessor<PsiElement> processor,
                                              String titlePattern,
                                              PsiElement[] elements) {
    if (TargetElementUtilBase.inVirtualSpace(editor, offset)) {
      return false;
    }

    final PsiReference reference = TargetElementUtilBase.findReference(editor, offset);        // I'm getting null here

    if (elements == null || elements.length == 0) {
      final Collection<PsiElement> candidates = suggestCandidates(reference);
      elements = PsiUtilCore.toPsiElementArray(candidates);
    }

    if (elements.length == 1) {
      PsiElement element = elements[0];
      LOG.assertTrue(element != null);
      processor.execute(element);
      return true;
    }
    if (elements.length > 1) {
      final TextRange range = reference.getRangeInElement();                                 // the IDE is not checking for null here !!
      final String refText = range.substring(reference.getElement().getText());              // neither here 
      String title = MessageFormat.format(titlePattern, refText);
      NavigationUtil.getPsiElementPopup(elements, new DefaultPsiElementCellRenderer(), title, processor).showInBestPositionFor(editor);
      return true;
    }
    return false;
  }

I've tried adding an extension point to "psi.referenceContributor" but that code is executed too often, that's why I choose the GotoDeclarationHandler, I want to add options to that part only (at really to GotoDeclarationAction only).

Do you know a better solution for this?
and if not, do you know how to check if it's safe to show the popup?

Thanks !!

IntelliJ IDEA 12 does have a null check in this code. Generally, psi.referenceContributor is the correct way to implement such behavior, as it lets you support many other features beyond goto definition (completion, find usages, rename etc.).

The problem is that the psiReference extension point is executed too often, and I need to run very slow code, but I only need to do that when an GotoDeclarationAction is executed, can I check for that? in the psiReference or in the GotoDeclarationHandler.

The correct solution is to cache the results of your very slow calculation.

I'm caching it, but if the user changes some xml file (and this happens very often too!) I need to calculate the gotodeclaration again (and sometimes I need to run PHP code for getting the right result !)

PS: I think would be a good idea to have extensions points for every action, so if we want add some functionality to an existent and specific action we just go for it, receive the original event, and decide if the default action should execute too or stop after our custom action.