Run composer require checker and solve issues.

src/Mapper/Layer/MarkersLayerMapper.php

@@ -170,7 +170,7 @@ class MarkersLayerMapper extends AbstractLayerMapper implements GeoJsonMapper
      * @param Model   $model   The layer model.
      * @param Request $request Optional building request.
      *
-     * @return \Model\Collection|null
+     * @return \Contao\Model\Collection|null
      */
     protected function loadMarkerModels(Model $model, Request $request = null)
     {

src/Mapper/Layer/OverpassLayerMapper.php

@@ -13,6 +13,7 @@
 namespace Netzmacht\Contao\Leaflet\Mapper\Layer;
 
 use Contao\Model;
+use Contao\StringUtil;
 use Netzmacht\Contao\Leaflet\Definition\Layer\OverpassLayer;
 use Netzmacht\Contao\Leaflet\Mapper\DefinitionMapper;
 use Netzmacht\Contao\Leaflet\Mapper\Request;
@@ -146,7 +147,7 @@ class OverpassLayerMapper extends AbstractLayerMapper
      */
     private function filterAmenityIconsConfig($amenityIconsConfig)
     {
-        $amenityIconsConfig = deserialize($amenityIconsConfig, true);
+        $amenityIconsConfig = StringUtil::deserialize($amenityIconsConfig, true);
         $amenityIconsMap    = [];
 
         foreach ($amenityIconsConfig as $config) {

src/Mapper/Layer/VectorsLayerMapper.php

@@ -188,11 +188,11 @@ class VectorsLayerMapper extends AbstractLayerMapper implements GeoJsonMapper
      * Add javascript callbacks.
      *
      * @param GeoJson $definition The definition.
-     * @param \Model  $model      The database model.
+     * @param Model   $model      The database model.
      *
      * @return void
      */
-    protected function addCallbacks(GeoJson $definition, \Model $model)
+    protected function addCallbacks(GeoJson $definition, Model $model)
     {
         if ($model->pointToLayer) {
             $definition->setPointToLayer(new Expression($model->pointToLayer));