'Network Error' when creating membership type from contact membership tab

[vinuvarshith]

Overview

'Network Error' when creating membership type from contact membership tab.

To replicate:
Create a contact -> go to membership tab -> new membership
Select membership organisation and type -> choose priceset
Then select manual membership and price from Membership Organisation and Type
At this point the network error and invalid entity filter warning appears

Technical Details

In the above form null is used as 'null' (with quotes) which is not actually null because it is treated as a string with value 'null'.

[civibot]

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[colemanw]

Unrelated fail. @civicrm-builder retest this please.

[jitendrapurohit]

The change done in this PR fixes the issue.

Noting that we also use null string to set subtype to NULL at the form level. Eg https://github.com/civicrm/civicrm-core/blob/master/CRM/Custom/Form/CustomData.php#L100

I tested by creating multiple custom fields for different membership type and altered the selection on new membership form. The custom fields were shown/hidden properly. Confirming that I don't find any problems with this change of 'null' to null. Hence approving it from my side.

[jitendrapurohit]

test this please

[mattwire]

@jitendrapurohit has reviewed and tested. I've looked at the code and agree with the changes - ok to merge.

[jitendrapurohit]

ping @eileenmcnaughton