Add fix from commit r58 "fixed subtle bug related to opening file in
untouched widget":
https://sourceforge.net/p/wavesurfer/code/58/#diff-2

--- src/wsurf/wavebar__orig.tcl	2018-09-08 18:34:58.049674304 +0200
+++ src/wsurf/wavebar.tcl	2018-09-08 18:27:15.450901639 +0200
@@ -747,6 +747,11 @@
 
 proc wavebar::_update {w {width -1} {height -1}} {
  upvar [namespace current]::${w}::data d
+
+    if {![winfo exists $w]} {
+	return
+    }
+
  if {$width>0} {set d(c0width) $width}
  if {$height>0} {set d(c0height) $height}
  if {$d(c0width)<=0} return