No topic selected error not handled in data_loader_widget

[romainreignier]

If one does not select a topic, the selected row is -1 and the .text() throws an exception here:

qgis_ros/src/qgis_ros/ui/data_loader_widget.py

Lines 54 to 55 in 1789d5b

	row = self.tableWidget.currentRow()
	topicName = self.tableWidget.item(row, 0).text()

There should be a check if row == -1 or catch the exception.

For now, the Add Layer button stays disabled and with the Loading... text on the bag dialog.

I am not a Python developer so I am not sure how to best catch the error.