title: StackOverflowError While Adding FormPage to FormEditor
body: <p>I have a really basic <code>FormEditor</code> and <code>FormPage</code>:</p>
  
  <pre><code>public class CustomFieldSetVersionEditor extends FormEditor {
  
      @Override
      protected void addPages() {
          try {
              addPage(new MyFormPage(this), getEditorInput());
          } catch (final PartInitException ex) {
              ex.printStackTrace();
          }
      }
  
      @Override
      public void doSave(final IProgressMonitor monitor) {}
  
      @Override
      public void doSaveAs() {}
  
      @Override
      public boolean isSaveAsAllowed() { return false; }
  }
  
  class MyFormPage extends FormPage {
  
      public MyFormPage(final FormEditor editor) {
          super(editor, "id", "Title");
      }
  
      @Override
      protected void createFormContent(final IManagedForm managedForm) { }
  
  }
  </code></pre>
  
  <p>This admittedly complex and exotic use case brings the following exception:</p>
  
  <blockquote>
    <p>java.lang.StackOverflowError
      at org.eclipse.swt.widgets.Widget.getData(Widget.java:525)
      at org.eclipse.ui.part.MultiPageEditorPart.getEditor(MultiPageEditorPart.java:599)
      at org.eclipse.ui.part.MultiPageEditorPart.getActiveEditor(MultiPageEditorPart.java:523)
      at org.eclipse.ui.forms.editor.FormEditor.getActiveEditor(FormEditor.java:430)
      at org.eclipse.ui.forms.editor.FormEditor$FormEditorSelectionProvider.getSelection(FormEditor.java:84)
      at org.eclipse.ui.forms.editor.FormEditor$FormEditorSelectionProvider.getSelection(FormEditor.java:89)
      at org.eclipse.ui.forms.editor.FormEditor$FormEditorSelectionProvider.getSelection(FormEditor.java:89)
      at org.eclipse.ui.forms.editor.FormEditor$FormEditorSelectionProvider.getSelection(FormEditor.java:89)
      ...</p>
  </blockquote>
  
  <p>However, if I use <code>addPage(IFormPage)</code> instead of  <code>addPage(IEditorPart, IEditorInput)</code> it works. There is even a "bug" report for this problem: <a href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=283039" rel="nofollow">Bug 283039</a> (it's no bug, because... there is no reason given).</p>
  
  <p>We don't need to discuss if this is a bug, but what I want to know is: How do I add a <code>FormPage</code> to a <code>FormEditor</code> while still defining the <code>IEditorInput</code>?</p>