The issue solves itself after I rewrote my code. Here is my final code, but I don't think it has really changed... Don't really understand what happened...
private Widget getContainer() {
SimpleContainer sc = new SimpleContainer();
BorderLayoutContainer blc = new BorderLayoutContainer();
ContentPanel validationView = new ContentPanel();
validationView.setHeadingText("Validation");
ContentPanel center = new ContentPanel();
center.setHeaderVisible(false);
center.add(editorMVP.getDisplay());
blc.setCenterWidget(center);
BorderLayoutData southData = new BorderLayoutData(250);
southData.setCollapsible(true);
southData.setSplit(false);
southData.setCollapsed(true);
blc.setSouthWidget(validationView, southData);
blc.collapse(LayoutRegion.SOUTH);
sc.add(blc);
return sc.asWidget();
}