Update app.py

app.py

@@ -42,6 +42,8 @@ def calculator(input_text):
         return f"Error: {str(e)}"
 
 def append_to_input(current, value):
+    if current is None:
+        current = ""
     if value in ['+', '-', '*', '/']:
         return current + f" {value} "
     return current + value
@@ -50,6 +52,8 @@ def clear_input(current):
     return ""
 
 def backspace(current):
+    if current is None:
+        return ""
     return current[:-1]
 
 class CalculationInput(BaseModel):