Fix error when using upload button more than once

src/qml/Chat/Composer.qml

@@ -216,6 +216,7 @@ Rectangle {
             HFileDialogOpener {
                 dialog.title: qsTr("Select files to upload")
                 onFileChanged: {
+                    if (! file) return
                     let path = Qt.resolvedUrl(file).replace(/^file:/, "")
                     let args = [chatPage.roomId, path]
                     py.callClientCoro(chatPage.userId, "send_file", args)