TypeError: b2a_base64() argument 1 must be string or buffer, not None

*I'm submitting a ...*
- [ x] bug report
- [ ] feature request
- [ ] support request

## General infomration
After putting in ESN and username/password, after restart of Kodi I get this error
TypeError: b2a_base64() argument 1 must be string or buffer, not None

### Prerequisites

* [ ] Are you running the latest version?
KODI 18.0 RC5
plugin version 13.21
_Also tried the 14.0 beta, but then I get the error:
MSLError: Widevine CryptoSession getKeyRequest failed!
(full log for that error: http://paste.kodi.tv/iqetovodog)_

### Description

[Description of the bug or feature]

### Steps to Reproduce

1. install addon
2. fill in required esn & pw/un
3. restart kodi, get this error

**Expected behavior:** [What you expected to happen]
play videos in netflix addon
**Actual behavior:** [What actually happened]
cannot open the addon
## Context (Environment)

### Installation

* [ ] I installed the plugin via zip from the Releases page
* [ x] I´m using the Netflix Repo
* [ ] I´m using a different source (Please tell which)

### Operating System

* [ ] Linux (x86/x64)
* [ ] OSX (x86/x64)
* [ ] Windows (x86/x64)
* [ ] Linux (ARM)
* [x ] Android

#### Additional informatin on the environment
Amlogic SKYSTREAM_3 with Android 8.1.0 API level 27, kernel: Linux ARM 32-bit version 4.9.76
ARMv8 Processor rev 4 (v8l)
DRMinfo: L3

## Debug log

http://paste.kodi.tv/ixuzojufic

Start by reading the linked debug log and reproducing the failure on Kodi 18.0 RC5 with plugin 13.21 on the reported Android device. Trace the b2a_base64() error and verify the addon opens and can play videos after the fix; the Widevine error from version 14.0 is a separate condition to compare.

Written by the indexing model from the issue text.