Double check the geometry type returned by your line object in QGIS 3.8. The xAt() method belongs to the QgsLineString class and won't work with a QgsMultiLineString object.
You can easily access the x coordinate of the first vertex of your line features like this:
layer = iface.activeLayer()
all_features = layer.getFeatures()
You can directly produce features of concentric buffers as polygons by using linspace method from numpy python module. As you have in your attributes table 1440 features, I know that, in your case, polygon sides number is 360 for each one of your concentric buffers. Algorithm for extracting points for each polygon features is easily inferred; as it can be ...