Consistently check for type mismatch in all ports in wxDataViewCtrl.

Move the checks for the type mismatch between the type of the value returned
by wxDataViewModel and the type expected by wxDataViewRenderer into common
code. This avoids duplicating the same code in wxGTK and wxOSX and, more
importantly, means that this check is also performed in wxMSW when using the
generic version, so that the problems such as the one fixed in 3ff8c3c ("add
missing wxDataViewDateRenderer::GetDefaultType()") would be visible there too.

src/common/datavcmn.cpp

@@ -683,8 +683,7 @@ bool wxDataViewRendererBase::StartEditing( const wxDataViewItem &item, wxRect la
     m_item = item; // remember for later
 
     unsigned int col = GetOwner()->GetModelColumn();
-    wxVariant value;
-    dv_ctrl->GetModel()->GetValue( value, item, col );
+    const wxVariant& value = CheckedGetValue(dv_ctrl->GetModel(), item, col);
 
     m_editorCtrl = CreateEditorCtrl( dv_ctrl->GetMainWindow(), labelRect, value );
 
@@ -778,13 +777,57 @@ bool wxDataViewRendererBase::FinishEditing()
     return false;
 }
 
+wxVariant
+wxDataViewRendererBase::CheckedGetValue(const wxDataViewModel* model,
+                                        const wxDataViewItem& item,
+                                        unsigned column) const
+{
+    wxVariant value;
+    model->GetValue(value, item, column);
+
+    // We always allow the cell to be null, regardless of the renderer type.
+    if ( !value.IsNull() )
+    {
+        if ( value.GetType() != GetVariantType() )
+        {
+            // If you're seeing this message, this indicates that either your
+            // renderer is using the wrong type, or your model returns values
+            // of the wrong type.
+            wxLogDebug("Wrong type returned from the model for column %u: "
+                       "%s required but actual type is %s",
+                       column,
+                       GetVariantType(),
+                       value.GetType());
+
+            // Don't return data of mismatching type, this could be unexpected.
+            value.MakeNull();
+        }
+    }
+
+    return value;
+}
+
+bool
+wxDataViewRendererBase::PrepareValue(const wxDataViewModel* model,
+                                     const wxDataViewItem& item,
+                                     unsigned column)
+{
+    const wxVariant& value = CheckedGetValue(model, item, column);
+
+    if ( value.IsNull() )
+        return false;
+
+    SetValue(value);
+
+    return true;
+}
+
 void wxDataViewRendererBase::PrepareForItem(const wxDataViewModel *model,
                                             const wxDataViewItem& item,
                                             unsigned column)
 {
-    wxVariant value;
-    model->GetValue(value, item, column);
-    SetValue(value);
+    if ( !PrepareValue(model, item, column) )
+        return;
 
     wxDataViewItemAttr attr;
     model->GetAttr(item, column, attr);