There is an easy work around to use the paint bucket on another layer by following the below steps:
Step 1 - Duplicate the layer containing the line art.
Step 2 - Hide the top layer so that you are only viewing the bottom layer.
Step 3 - Choose the paint bucket and the color that you want to use and apply it to the object that you want to fill.
Step 4 - Zoom in and select the object outline to remove the outline.
Step 5 - Unhide the layer that you hid.
This will give you a layer with your object art and a separate layer with your color.