[GRASS-dev] gis.m: error message when trying to add layer without open map display

[Moritz Lennert]

Michael,

Since you have been busy working on error catching in gis.m: I've always 
found that the error that occurs when you try to add a layer even though 
there is no open map display quite difficult to understand for newbies:

invalid command name ".mainframe.frame.pw1.f0.frame.pgs.fpage_1.sw.tree_1"
invalid command name ".mainframe.frame.pw1.f0.frame.pgs.fpage_1.sw.tree_1"
     while executing
"$tree($mon) selection get"
     (procedure "GmTree::add" line 11)
     invoked from within
"GmTree::add vector"
     ("uplevel" body line 1)
     invoked from within
"uplevel \#0 $cmd"
     (procedure "Button::_release" line 18)
     invoked from within
"Button::_release .mainframe.topf.tb0.bbox2.b0"
     (command bound to event)


Would it be possible to catch this and produce a nicer error message 
such as: "You need to open a map display first." ?

Moritz