Linux specific: Fixed ObjectDataViewModel::GetColumnType()

When "string" type was returned, strange editing TextControl was appeared.
+ Added check of the selection for ObjectList::toggle_printable_state() function

src/slic3r/GUI/GUI_ObjectList.cpp

@@ -3909,6 +3909,8 @@ void ObjectList::toggle_printable_state()
 {
     wxDataViewItemArray sels;
     GetSelections(sels);
+    if (sels.IsEmpty())
+        return;
 
     wxDataViewItem frst_item = sels[0];