Update app/materials_library.py

app/materials_library.py

@@ -1231,7 +1231,7 @@ def validate_material(
     if (not edit_mode or name != original_name) and name in st.session_state.project_data["materials"]["project"]:
         return False, f"Material name '{name}' already exists in project materials."
     
-    if category is not in MATERIAL_CATEGORIES:
+    if category not in MATERIAL_CATEGORIES:
         return False, "Invalid material category."
     
     if thermal_conductivity <= 0: