Fix NPE when viewer control doesn't adapt to CompareConfiguration

Follow up to
#2629

which introduced an NPE when doing refactor->rename in CDT:
eclipse-cdt/cdt#1172

Author: jonahgraham

bundles/org.eclipse.ltk.ui.refactoring/src/org/eclipse/ltk/internal/ui/refactoring/PreviewWizardPage.java

@@ -673,7 +673,9 @@ private void showPreview(PreviewNode element) {
 						if (container != null) {
 							if (newViewer.getControl() instanceof IAdaptable adaptable) {
 								CompareConfiguration config = adaptable.getAdapter(CompareConfiguration.class);
-								config.setContainer(new WizardCompareContainer(container));
+								if (config != null) {
+									config.setContainer(new WizardCompareContainer(container));
+								}
 							}
 						}
 					} else {