I am binding a textbox to an object, like so:
The default constructor is required for a type to be instantiated in XAML. As a workaround you can simply create a subclass of TaskVM that will have the default contructor and use it as a design time data context.
Another alternative is to set d:IsDesignTimeCreatable to False and a substitute type will be created for you at runtime (using your TaskVM type as a "shape").