[save-selected] get sheet names from selectedRows

Only saving selectedRows, so the sheet names should be pulled from
there.

Not all IndexSheets carry their sub-sheets in their `source`.

Closes #1339

visidata/save.py

@@ -163,6 +163,6 @@ def save_txt(vd, p, *vsheets):
 
 BaseSheet.addCommand('^S', 'save-sheet', 'vd.saveSheets(inputPath("save to: ", value=getDefaultSaveName()), sheet, confirm_overwrite=options.confirm_overwrite)', 'save current sheet to filename in format determined by extension (default .tsv)')
 BaseSheet.addCommand('g^S', 'save-all', 'vd.saveSheets(inputPath("save all sheets to: "), *vd.stackedSheets, confirm_overwrite=options.confirm_overwrite)', 'save all sheets to given file or directory)')
-IndexSheet.addCommand('g^S', 'save-selected', 'vd.saveSheets(inputPath("save %d sheets to: " % nSelectedRows, value="_".join(vs.name or "blank" for vs in source)), *selectedRows, confirm_overwrite=options.confirm_overwrite)', 'save all selected sheets to given file or directory')
+IndexSheet.addCommand('g^S', 'save-selected', 'vd.saveSheets(inputPath("save %d sheets to: " % nSelectedRows, value="_".join(getattr(vs, "name", None) or "blank" for vs in selectedRows)), *selectedRows, confirm_overwrite=options.confirm_overwrite)', 'save all selected sheets to given file or directory')
 Sheet.addCommand('', 'save-col', 'save_cols([cursorCol])', 'save current column only to filename in format determined by extension (default .tsv)')
 Sheet.addCommand('', 'save-col-keys', 'save_cols(keyCols + [cursorCol])', 'save key columns and current column to filename in format determined by extension (default .tsv)')