Magento – Magento 2 : Admin form using ui component not able to do edit and save

I have created an admin form using ui component and I could save a new form using the following code. But now I am facing an issue with edit form save.

ext_form.xml

<fieldset name="ext_details">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <item name="collapsible" xsi:type="boolean">true</item>
            <item name="label" xsi:type="string" translate="true">Ext [General]</item>
            <item name="sortOrder" xsi:type="number">10</item>
            <item name="openOnShow" xsi:type="boolean">true</item>
        </item>
    </argument>
    <field name="ext_id">
        <argument name="data" xsi:type="array">
            <item name="config" xsi:type="array">
                <item name="visible" xsi:type="boolean">false</item>
                <item name="dataType" xsi:type="string">text</item>
                <item name="formElement" xsi:type="string">input</item>
                <item name="source" xsi:type="string">ext_id</item>
            </item>
        </argument>
    </field>
    <field name="product_title">
        <argument name="data" xsi:type="array">
            <item name="config" xsi:type="array">
                <item name="label" xsi:type="string">Ext Title</item>
                <item name="visible" xsi:type="boolean">true</item>
                <item name="dataType" xsi:type="string">text</item>
                <item name="disabled" xsi:type="boolean">false</item>
                <item name="formElement" xsi:type="string">input</item>
                <item name="source" xsi:type="string">title</item>
                <item name="validation" xsi:type="array">
                    <item name="required-entry" xsi:type="boolean">true</item>
                </item>
            </item>
        </argument>
    </field>
    <field name="status">
        <argument name="data" xsi:type="array">
            <item name="options" xsi:type="object">Vendorname\Modulename\Model\Config\Source\Status</item>
            <item name="config" xsi:type="array">
                <item name="label" xsi:type="string" translate="true">Status</item>
                <item name="visible" xsi:type="boolean">true</item>
                <item name="dataType" xsi:type="string">number</item>
                <item name="formElement" xsi:type="string">select</item>
                <item name="source" xsi:type="string">status</item>
                <item name="dataScope" xsi:type="string">status</item>
                <item name="validation" xsi:type="array">
                    <item name="required-entry" xsi:type="boolean">true</item>
                </item>
            </item>
        </argument>
    </field>
</fieldset> 

save.php [Updated]

namespace Vendorname\Modulename\Controller\Adminhtml\Modulename;

use Magento\Framework\App\Filesystem\DirectoryList;

class Save extends \Vendorname\Modulename\Controller\Adminhtml\Modulename
{
    protected $_fileUploaderFactory;
    /**
    * @var \Magento\Framework\Filesystem
    */
    protected $filesystem;

    /**
     * @var \Magento\Backend\Helper\Js
     */
    protected $_jsHelper;



    /**
     * Ext Model
     * @var Vendorname\Modulename\Model\Modulename
     */
    protected $extModel;

    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \Magento\Backend\Helper\Js $jsHelper,
        \Magento\MediaStorage\Model\File\UploaderFactory $fileUploaderFactory,
        \Magento\Framework\Filesystem $filesystem,
        \Magento\Framework\Registry $coreRegistry,
        \Vendorname\Modulename\Model\ImageUploader $imageUploader, 
        \Vendorname\Modulename\Model\ExtFactory $extModel      
    )
    {
        $this->_jsHelper = $jsHelper;       
        $this->_fileUploaderFactory = $fileUploaderFactory;
        $this->filesystem = $filesystem;
        $this->imageUploader = $imageUploader;
        $this->extModel = $extModel;
        parent::__construct($context,$coreRegistry);
    }   

    /**
     * Save action
     *
     * @return \Magento\Framework\Controller\ResultInterface
     */
    public function execute()
    {
        $data = $this->getRequest()->getPostValue();
        /** @var \Magento\Backend\Model\View\Result\Redirect $resultRedirect */
        $resultRedirect = $this->resultRedirectFactory->create();
        if ($data) {  

            $model = $this->extModel->create();

            $id = $this->getRequest()->getParam('ext_id');
            if ($id) {
                $model->load($id);
            }

            $data = array_filter($data, function($value) { return $value !== ''; });
            $model->setData($data);

            try {
                $model->save();

                $this->messageManager->addSuccess(__('You saved this item.'));
                $this->_objectManager->get('Magento\Backend\Model\Session')->setFormData(false);
                if ($this->getRequest()->getParam('back')) {
                    return $resultRedirect->setPath('*/*/edit', ['ext_id' => $model->getId(), '_current' => true]);
                }
                return $resultRedirect->setPath('*/*/');
            } catch (\Magento\Framework\Exception\LocalizedException $e) {
                $this->messageManager->addError($e->getMessage());
            } catch (\RuntimeException $e) {
                $this->messageManager->addError($e->getMessage());
            } catch (\Exception $e) {
                $this->messageManager->addException($e, __('Something went wrong while saving the ext.'));
            }

            $this->_getSession()->setFormData($data);
            return $resultRedirect->setPath('*/*/edit', ['ext_id' => $this->getRequest()->getParam('ext_id')]);
        }
        return $resultRedirect->setPath('*/*/');
    }

Now upon edit and save action I am getting an error

Something went wrong while saving the ext.