f error if no active signing session

Author: dunxen

lightning/src/ln/channel.rs

@@ -7785,6 +7785,13 @@ where
 				debug_assert!(false, "We don't allow contributing inputs that are not spending P2WPKH, P2WSH, or P2TR");
 				return Err(APIError::APIMisuseError { err: format!("Input at index {input_idx} does not spend from one of P2WPKH, P2WSH, or P2TR") });
 			}
+		} else {
+			return Err(APIError::APIMisuseError {
+				err: format!(
+					"No active signing session for channel with ID {}",
+					self.context.channel_id
+				),
+			});
 		}
 		Ok(())
 	}