Bug 182190 [StatusHandling] [Wizards] getControl assert on wizard page has no message
M6 If you forget to set the control on your wizard page our assert gives no feedback. We need an error message.
bundles/org.eclipse.jface/src/org/eclipse/jface/wizard/WizardDialog.java
* @param enableCancelButton <code>true</code> if the Cancel button should
     *   be enabled, and <code>false</code> if it should be disabled
     * @return the saved UI state
     */
    private Object aboutToStart(boolean enableCancelButton) {
        Map savedState = null;
        if (getShell() != null) {
            
            Control focusControl = getShell().getDisplay().getFocusControl();
            if (focusControl != null && focusControl.getShell() != getShell()) {
				focusControl = null;
			}
            boolean needsProgressMonitor = wizard.needsProgressMonitor();
            cancelButton.removeSelectionListener(cancelListener);
            
            Display d = getShell().getDisplay();
            waitCursor = new Cursor(d, SWT.CURSOR_WAIT);
            setDisplayCursor(waitCursor);
            
            arrowCursor = new Cursor(d, SWT.CURSOR_ARROW);
            cancelButton.setCursor(arrowCursor);
            
            savedState = saveUIState(needsProgressMonitor && enableCancelButton);
            if (focusControl != null) {
				savedState.put(FOCUS_CONTROL, focusControl);
			}
            
            if (needsProgressMonitor) {
                progressMonitorPart.attachToCancelComponent(cancelButton);
                progressMonitorPart.setVisible(true);