This article is a solution If you face an uncaught TypeError which details

explode() expects parameter 2 to be string, null given in /vendor/gene/module-braintree/Model/Lpm/Config.php:127

All you need to do is check the following code & replace the method getAllowedMethods in /vendor/gene/module-braintree/Model/Lpm/Config.php.

public function getAllowedMethods(): array
{
    $this->allowedMethods = [];
    $allowedMethods = [];
    if ($this->getValue(self::KEY_ALLOWED_METHODS,$this->storeConfigResolver->getStoreId()))
    {
	$allowedMethods = explode(',',$this->getValue(self::KEY_ALLOWED_METHODS,$this->storeConfigResolver->getStoreId()));
    }
    foreach ($allowedMethods as $allowedMethod) {
        $this->allowedMethods[] = [
            'method' => $allowedMethod,
            'label' => constant('self::LABEL_'.strtoupper($allowedMethod)),
            'countries' => constant('self::COUNTRIES_'.strtoupper($allowedMethod))
        ];
    }
    return $this->allowedMethods;
}