Update app.py

app.py

@@ -111,13 +111,14 @@ def create_bibtex_entry(data):
     if year and 'year' not in data:
         data['year'] = year
 
-    match_year = re.search(r'(\d{4})', data['year'])
-    
-    if match_year:
-        data['year'] = match_year.group(1)
-        year = data['year']
-    else:
-        data.pop('year', '')
+    if "year" in data:
+        match_year = re.search(r'(\d{4})', data['year'])
+        
+        if match_year:
+            data['year'] = match_year.group(1)
+            year = data['year']
+        else:
+            data.pop('year', '')
 
     #Pages conformity.
     if 'pages' in data: