[QRCode Android] Fix crash getting current url.

Bug: 1085209
Change-Id: I9868b9a7012e9dc7d23a710fea21186d8b73107a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210798Reviewed-by: Tanya Gupta <tgupta@chromium.org>
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770828}

chrome/browser/share/android/java/src/org/chromium/chrome/browser/share/qrcode/share_tab/QrCodeShareMediator.java

@@ -20,6 +20,7 @@ import org.chromium.chrome.R;
 import org.chromium.chrome.browser.ChromeActivity;
 import org.chromium.chrome.browser.share.SaveImageNotificationManager;
 import org.chromium.chrome.browser.share.qrcode.QRCodeGenerationRequest;
+import org.chromium.chrome.browser.tab.Tab;
 import org.chromium.components.browser_ui.share.ShareImageFileUtils;
 import org.chromium.ui.modelutil.PropertyModel;
 import org.chromium.ui.widget.Toast;
@@ -49,8 +50,11 @@ class QrCodeShareMediator implements ShareImageFileUtils.OnImageSaveListener {
 
         // TODO(crbug.com/1083351): Get URL from Sharing Hub.
         if (context instanceof ChromeActivity) {
-            mUrl = ((ChromeActivity) context).getActivityTabProvider().get().getUrl().getSpec();
-            refreshQrCode(mUrl);
+            Tab tab = ((ChromeActivity) context).getActivityTabProvider().get();
+            if (tab != null) {
+                mUrl = tab.getUrl().getSpec();
+                refreshQrCode(mUrl);
+            }
         }
     }