Fixes T49855 - Exception when there is no line set

The UI code assumed the existence of an active lineset. Turns out that
the final lineset can be removed (unlike render layers, where there is
always at least one).

render_freestyle_svg.py

@@ -190,19 +190,28 @@ class SVGExporterLinesetPanel(bpy.types.Panel):
         scene = context.scene
         svg = scene.svg_export
         freestyle = scene.render.layers.active.freestyle_settings
-        linestyle = freestyle.linesets.active.linestyle
 
-        layout.active = (svg.use_svg_export and freestyle.mode != 'SCRIPT')
-        row = layout.row()
-        column = row.column()
-        column.prop(linestyle, 'use_export_strokes')
+        try:
+            linestyle = freestyle.linesets.active.linestyle
 
-        column = row.column()
-        column.active = svg.object_fill
-        column.prop(linestyle, 'use_export_fills')
+        except AttributeError:
+            # Linestyles can be removed, so 0 linestyles is possible.
+            # there is nothing to draw in those cases.
+            # see https://developer.blender.org/T49855
+            return
 
-        row = layout.row()
-        row.prop(linestyle, "stroke_color_mode", expand=True)
+        else:
+            layout.active = (svg.use_svg_export and freestyle.mode != 'SCRIPT')
+            row = layout.row()
+            column = row.column()
+            column.prop(linestyle, 'use_export_strokes')
+
+            column = row.column()
+            column.active = svg.object_fill
+            column.prop(linestyle, 'use_export_fills')
+
+            row = layout.row()
+            row.prop(linestyle, "stroke_color_mode", expand=True)
 
 
 class SVGExport(bpy.types.PropertyGroup):