The error Cannot use [] for reading is related to PHP < 7.0.
IP is fully working without an problems while using PHP 7.0 or 7.1 but throws the error while running PHP 5.6.
The bug will be fixed within the next release. To hitfix this, open the file application/modules/custom_fields/models/Mdl_custom_fields.php and replace line 257 with the following line:
array_push($field->$field_id_fieldlabel, $custom_value->custom_values_value);
Fore reference: ticket is https://development.invoiceplane.com/browse/IP-574
This issue was already solved in pull request #538. PLEASE DO NOT OPEN NEW PULL REQUESTS!