Fix null access error

Author: crokita

lib/js/src/manager/screen/utils/_PresentAlertOperation.js

@@ -455,6 +455,13 @@ class _PresentAlertOperation extends _Task {
      * @returns {Boolean} - True if soft button images are supported, false if not.
      */
     supportsSoftButtonImages () {
+        if (this._currentWindowCapability === null
+            || this._currentWindowCapability === undefined
+            || this._currentWindowCapability.getSoftButtonCapabilities() === null
+            || this._currentWindowCapability.getSoftButtonCapabilities() === undefined
+            || this._currentWindowCapability.getSoftButtonCapabilities().length === 0) {
+            return true; // return true if non-existant soft button capability
+        }
         const softButtonCapabilities = this._currentWindowCapability.getSoftButtonCapabilities()[0];
         return softButtonCapabilities.getImageSupported();
     }