Fix type cast using 'whereType' instead of 'as' operator

[bikcrum]

Description

This is the minimal fix for typecasting error caused in null safety mode.

Implementation:

Issue:

<Widget?>[....].where((a) => a != null).toList() as List<Widget>

// causes type 'List<Widget?>' is not a subtype of type 'List<Widget>' in type cast

Fix:

<Widget?>[....].where((a) => a != null).whereType<Widget>().toList()

Effective line numbers: [435-437], [600-620]

Related Issues

#69 #70 #71 #72

[housecode]

is it ready to use at production ?

[definitelyme]

@Dn-a can this be merged?