FIX: Preview trows error if ora is selected as output format.

Preview cannot be used for ORA as it is only intended for format
transformation. We disable the preview button to prevent user
from using an unsupported feature

artscript2.tcl

@@ -930,7 +930,7 @@ proc guiFileList { w } {
 proc guiThumbnail { w } {
 	ttk::labelframe $w.thumb -width 276 -height 316 -padding 6 -labelanchor n -text "Thumbnail"
 	set ::widget_name(thumb-im) [ttk::label $w.thumb.im -anchor center -text "No Thumbnail"]
-	ttk::button $w.thumb.prev -text "Preview" -style small.TButton -command { showPreview }
+	set ::widget_name(thumb-prev) [ttk::button $w.thumb.prev -text "Preview" -style small.TButton -command { showPreview }]
 	pack $w.thumb.prev -side bottom
 }
 
@@ -1287,6 +1287,7 @@ proc setFormatOptions { w } {
 	# update listname
 	treeAlterVal {getOutputName $value $::outext $::ouprefix $::ousuffix} $::widget_name(flist) path output
 	$::widget_name(convert-but) configure -text "Convert" -command {convert}
+	$::widget_name(thumb-prev) state !disabled
 	switch -glob -- $::outext {
 		jpg	{
 			set ::iquality 92 
@@ -1308,6 +1309,7 @@ proc setFormatOptions { w } {
 			$w.qtb configure -text "Quality:"
 			$w.qal configure -from 0 -to 0
 			$::widget_name(convert-but) configure -text "Make ORA" -command {makeOra}
+			$::widget_name(thumb-prev) state disabled
 		}
 		webp { 
 			set ::iquality 100