Fix UI API called on a background thread

apache#551

src/ios/CDVCamera.m

@@ -185,16 +185,17 @@ - (void)takePicture:(CDVInvokedUrlCommand*)command
 
 - (void)showCameraPicker:(NSString*)callbackId withOptions:(CDVPictureOptions *) pictureOptions
 {
-    CDVCameraPicker* cameraPicker = [CDVCameraPicker createFromPictureOptions:pictureOptions];
-    self.pickerController = cameraPicker;
-
-    cameraPicker.delegate = self;
-    cameraPicker.callbackId = callbackId;
-    // we need to capture this state for memory warnings that dealloc this object
-    cameraPicker.webView = self.webView;
 
     // Perform UI operations on the main thread
     dispatch_async(dispatch_get_main_queue(), ^{
+        CDVCameraPicker* cameraPicker = [CDVCameraPicker createFromPictureOptions:pictureOptions];
+        self.pickerController = cameraPicker;
+
+        cameraPicker.delegate = self;
+        cameraPicker.callbackId = callbackId;
+        // we need to capture this state for memory warnings that dealloc this object
+        cameraPicker.webView = self.webView;
+
         // If a popover is already open, close it; we only want one at a time.
         if (([[self pickerController] pickerPopoverController] != nil) && [[[self pickerController] pickerPopoverController] isPopoverVisible]) {
             [[[self pickerController] pickerPopoverController] dismissPopoverAnimated:YES];