Disallow setting a blank alternative WebUI location

src/gui/optionsdialog.cpp

@@ -1250,6 +1250,10 @@ void OptionsDialog::on_buttonBox_accepted()
             m_ui->tabSelection->setCurrentRow(TAB_WEBUI);
             return;
         }
+        if (!isAlternativeWebUIPathValid()) {
+            m_ui->tabSelection->setCurrentRow(TAB_WEBUI);
+            return;
+        }
         m_applyButton->setEnabled(false);
         this->hide();
         saveOptions();
@@ -1269,6 +1273,10 @@ void OptionsDialog::applySettings(QAbstractButton *button)
             m_ui->tabSelection->setCurrentRow(TAB_WEBUI);
             return;
         }
+        if (!isAlternativeWebUIPathValid()) {
+            m_ui->tabSelection->setCurrentRow(TAB_WEBUI);
+            return;
+        }
         saveOptions();
     }
 }
@@ -1751,6 +1759,15 @@ bool OptionsDialog::webUIAuthenticationOk()
     return true;
 }
 
+bool OptionsDialog::isAlternativeWebUIPathValid()
+{
+    if (m_ui->groupAltWebUI->isChecked() && m_ui->textWebUIRootFolder->selectedPath().trimmed().isEmpty()) {
+        QMessageBox::warning(this, tr("Location Error"), tr("The alternative Web UI files location cannot be blank."));
+        return false;
+    }
+    return true;
+}
+
 void OptionsDialog::on_banListButton_clicked()
 {
     // call dialog window

src/gui/optionsdialog.h

@@ -172,6 +172,7 @@ private:
     bool setSslCertificate(const QByteArray &cert);
     bool schedTimesOk();
     bool webUIAuthenticationOk();
+    bool isAlternativeWebUIPathValid();
 
     QByteArray m_sslCert, m_sslKey;
 

src/webui/www/private/preferences_content.html

@@ -1667,8 +1667,14 @@
         settings.set('bypass_auth_subnet_whitelist', $('bypass_auth_subnet_whitelist_textarea').getProperty('value'));
 
         // Use alternative Web UI
-        settings.set('alternative_webui_enabled', $('use_alt_webui_checkbox').getProperty('checked'));
+        var alternative_webui_enabled = $('use_alt_webui_checkbox').getProperty('checked');
-        settings.set('alternative_webui_path', $('webui_files_location_textarea').getProperty('value'));
+        var webui_files_location_textarea = $('webui_files_location_textarea').getProperty('value');
+        if (alternative_webui_enabled && (webui_files_location_textarea.trim() === "")) {
+            alert("QBT_TR(The alternative Web UI files location cannot be blank.)QBT_TR[CONTEXT=OptionsDialog]");
+            return;
+        }
+        settings.set('alternative_webui_enabled', alternative_webui_enabled);
+        settings.set('alternative_webui_path', webui_files_location_textarea);
 
         settings.set('web_ui_clickjacking_protection_enabled', $('clickjacking_protection_checkbox').getProperty('checked'));
         settings.set('web_ui_csrf_protection_enabled', $('csrf_protection_checkbox').getProperty('checked'));