Fix image picker crash when used with android alarm manager (flutter#…

…1125)

Fixing [flutter/flutter#21972](flutter/flutter#21972)
Android Alarm Manager will create a background flutter view to register plugins. When that happens, image_picker will be registered without an activity, which then caused crash. 
So we escape the registration process when there is no activity in the registrar.

packages/image_picker/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.5.0+3
+
+* Fixed am Android crash when Image Picker is registered without an activity.
+
 ## 0.5.0+2
 
 * Log a more detailed warning at build time about the previous AndroidX

packages/image_picker/android/src/main/java/io/flutter/plugins/imagepicker/ImagePickerPlugin.java

@@ -21,6 +21,11 @@ public class ImagePickerPlugin implements MethodChannel.MethodCallHandler {
   private final ImagePickerDelegate delegate;
 
   public static void registerWith(PluginRegistry.Registrar registrar) {
+    if (registrar.activity() == null) {
+      // If a background flutter view tries to register the plugin, there will be no activity from the registrar,
+      // we stop the registering process immediately because the ImagePicker requires an activity.
+      return;
+    }
     final MethodChannel channel = new MethodChannel(registrar.messenger(), CHANNEL);
 
     final File externalFilesDirectory =

packages/image_picker/example/android/app/src/test/java/io/flutter/plugins/imagepicker/ImagePickerPluginTest.java

@@ -1,5 +1,6 @@
 package io.flutter.plugins.imagepicker;
 
+import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyZeroInteractions;
 import static org.mockito.Mockito.when;
@@ -95,6 +96,14 @@ public void onMethodCall_WhenSourceIsCamera_InvokesTakeImageWithCamera() {
     verifyZeroInteractions(mockResult);
   }
 
+  @Test
+  public void onResiter_WhenAcitivityIsNull_ShouldNotCrash() {
+    when(mockRegistrar.activity()).thenReturn(null);
+    ImagePickerPlugin.registerWith((mockRegistrar));
+    assertTrue(
+        "No exception thrown when ImagePickerPlugin.registerWith ran with activity = null", true);
+  }
+
   private MethodCall buildMethodCall(final int source) {
     final Map<String, Object> arguments = new HashMap<>();
     arguments.put("source", source);

packages/image_picker/pubspec.yaml

@@ -5,7 +5,7 @@ authors:
   - Flutter Team <flutter-dev@googlegroups.com>
   - Rhodes Davis Jr. <rody.davis.jr@gmail.com>
 homepage: https://github.com/flutter/plugins/tree/master/packages/image_picker
-version: 0.5.0+2
+version: 0.5.0+3
 
 flutter:
   plugin: