Update app.py

app.py

@@ -7,7 +7,10 @@ from androguard.misc import AnalyzeAPK
 from DroidDetective.DroidDetective import APK_Analyser
 
 def check_apk(apk_to_check):
-
+    
+    if apk_to_check == null:
+        return "Provide an APK to analyse."
+    
     output_builder = ""