fix/#802 Array fields error in form

[ibelar]

Fix for #802

Make sure reference is set when creating a Multiline in form using type array.

[codecov]

Codecov Report

Merging #804 into develop will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             develop     #804   +/-   ##
==========================================
  Coverage      76.19%   76.19%           
- Complexity      2287     2288    +1     
==========================================
  Files            119      119           
  Lines           5397     5397           
==========================================
  Hits            4112     4112           
  Misses          1285     1285

Impacted Files	Coverage Δ	Complexity Δ
src/Form.php	73.33% <100%> (ø)	69 <0> (+1)	⬆️

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[PhilippGrashoff]

LGTM. As someday there is another array field support planned (#752), some more has to happen here, but thats for another PR adressing #752

[acicovic]

@PhilippGrashoff you didn't merge this but I see no other reviewers. You think it should be reviewed by someone else before merging?