Import failing

Hi!

After the recent forced update I get an error for every import:

Object { key: "3ff1d8c7b1c33b119b289cae6dab218c", type: "notice", message: "Trying to access array offset on value of type null", file: "wp-content/plugins/ninja-forms/includes/Admin/Processes/ImportForm.php", line: 198, stack: (8) […], component: "Plugin: ninja-forms" }
?component: "Plugin: ninja-forms"
?file: "wp-content/plugins/ninja-forms/includes/Admin/Processes/ImportForm.php"
?key: "3ff1d8c7b1c33b119b289cae6dab218c"
?line: 198
?message: "Trying to access array offset on value of type null"
?stack: Array(8) [ "NF_Admin_Processes_ImportForm->cleanup()", "NF_Abstracts_BatchProcess->batch_complete()", "NF_Admin_Processes_ImportForm->startup()", … ]
??0: "NF_Admin_Processes_ImportForm->cleanup()"
??1: "NF_Abstracts_BatchProcess->batch_complete()"
??2: "NF_Admin_Processes_ImportForm->startup()"
??3: "NF_Abstracts_BatchProcess->init()"
??4: "NF_Abstracts_BatchProcess->__construct()"
??5: "NF_AJAX_REST_BatchProcess->post()"
??6: "NF_AJAX_REST_Controller->route()"
??7: "do_action('wp_ajax_nf_batch_process')"
??length: 8
??<prototype>: Array []
?type: "notice"
?<prototype>: Object { … }

fatal: decode_failed admin-import-export.js:48:16
    onCompleteCallback https://example.org/wp-content/plugins/ninja-forms/assets/js/admin-import-export.js?ver=5.5.9:48
    onCompleteCallback https://example.org/wp-content/plugins/ninja-forms/assets/js/admin-import-export.js?ver=5.5.9:42
    postToProcessor https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/batch-processor.js?ver=5.5.9:82
    jQuery 7
    postToProcessor https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/batch-processor.js?ver=5.5.9:74
    onOpenCallback https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/batch-processor.js?ver=5.5.9:60
    onOpen https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/ninjaModal.js?ver=5.5.9:176
    i https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/jBox.min.js?ver=5.5.9:2
    open https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/jBox.min.js?ver=5.5.9:2
    toggleModal https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/ninjaModal.js?ver=5.5.9:315
    NinjaModal https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/ninjaModal.js?ver=5.5.9:55
    NinjaBatchProcessor https://example.org/wp-content/plugins/ninja-forms/assets/js/lib/batch-processor.js?ver=5.5.9:116
    <anonym> https://example.org/wp-content/plugins/ninja-forms/assets/js/admin-import-export.js?ver=5.5.9:66
    jQuery 2

Any ideas why this happens and how to fix it?

All the best
Torsten