Fix null reference error in EmailTemplatePreview

- Add conditional rendering for previewTemplate to prevent null reference
- Add null checks in EmailTemplatePreview component for template properties
- Fix shortcodes filtering to handle null template properties
- Remove non-null assertion operator and use proper conditional rendering

Fixes: "Cannot read properties of null (reading 'email_subject')" error

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/components/admin/settings/NotifikasiTab.tsx

@@ -679,13 +679,15 @@ export function NotifikasiTab() {
       </Card>
 
       {/* Modal Email Preview */}
-      <EmailTemplatePreview
-        template={previewTemplate!}
-        open={isPreviewOpen}
-        onClose={() => setIsPreviewOpen(false)}
-        onTest={sendTestEmail}
-        isTestSending={testingTemplate === previewTemplate?.id}
-      />
+      {previewTemplate && (
+        <EmailTemplatePreview
+          template={previewTemplate}
+          open={isPreviewOpen}
+          onClose={() => setIsPreviewOpen(false)}
+          onTest={sendTestEmail}
+          isTestSending={testingTemplate === previewTemplate.id}
+        />
+      )}
     </div>
   );
 }