Revert "Fix warning during initialization"

[AlexHartford]

Reverts #236

Apparently DatePickerTheme was updated where backgroundColor is non-nullable, so now this PR created a warning instead of removing it.

./../../../.pub-cache/hosted/pub.dartlang.org/flutter_datetime_picker-1.5.1/lib/flutter_datetime_picker.dart:311:32: Warning: Operand of null-aware operation '??' has type 'Color' which excludes
null.
 - 'Color' is from 'dart:ui'.
                  color: theme.backgroundColor ?? Colors.white,

[AlexHartford]

Nevermind, it seems the original PR just didn't get deployed.

[hyouuu]

@AlexHartford without this PR I still see the issue - should we reopen this?

[AlexHartford]

@hyouuu No, the reason you still see the issue is that the package author never pushed a new version of the package containing the fix.

[hyouuu]

Gotcha guess I'll point to the git branch then. Thanks!