fix: add naive null check for minimapElement

Author: aminya

lib/minimap.js

@@ -1088,7 +1088,9 @@ export default class Minimap {
    * @return {DecorationManagement}
    */
   getDecorationManagement () {
-    if (this.DecorationManagement === undefined) {
+    if (this.DecorationManagement === undefined &&
+      this.minimapElement // TODO: find why this becomes null: https://github.com/atom-minimap/minimap/issues/766
+    ) {
       this.DecorationManagement = this.minimapElement.DecorationManagement
     }
     return this.DecorationManagement