Do not prefill a value for a checkbox

If the BooleanField in the given resource is not set, the tristate
logic would set it to false by default. This commit changes that to
differentiate between the value that is shown (false), and the
value that's actually present on the web.

Author: Tim Berners-Lee

src/widgets/forms.js

@@ -1202,20 +1202,22 @@ function buildCheckboxForm (dom, kb, lab, del, ins, form, store, tristate) { //
   }
   function refresh () {
     var state = holdsAll(ins)
+    var displayState = state
     if (del.length) {
       var negation = holdsAll(del)
       if (state && negation) {
-        box.appendChild(error.errorMessageBlock(dom,
+        box.appendChild(UI.widgets.errorMessageBlock(dom,
           'Inconsistent data in store!\n' + ins + ' and\n' + del))
         return box
       }
       if (!state && !negation) {
+        state = null
         let defa = kb.any(form, UI.ns.ui('default'))
-        state = defa ? defa.value === '1' : tristate ? null : false
+        displayState = defa ? defa.value === '1' : tristate ? null : false
       }
     }
     input.state = state
-    input.textContent = {true: checkMarkCharacter, false: cancelCharacter, null: dashCharacter}[input.state]
+    input.textContent = {true: checkMarkCharacter, false: cancelCharacter, null: dashCharacter}[displayState]
   }
 
   refresh()