MCW provides wrong value to getAttributesFromDca

[cgoIT]

The MultiColumnWizard calls the Method getAttributesFromDca. In one case it provides null as $context, in the other case $this is used as $context.

I think in both cases it should better be $this->objDca.

See also:
inspiredminds/contao-extended-form-fields#8
cgoIT/calendar-extended-bundle#46

[cgoIT]

btw.: If you agree to the proposed solution and if you want me to I can also provide a PR for that.