iOS - Fix the crash of pageview controller (#1292)

* LEARNER-7377 * Remove a retainCycle.
Enable animations for the transition of views between the units
Letting the view to complete its lifecycle before performing any extensive UI operations. It was causing the crash because the os wasn’t getting a true picture of view on which it was supposed to perform extensive UI operations.

Source/CourseContentPageViewController.swift

@@ -77,8 +77,8 @@ public class CourseContentPageViewController : UIPageViewController, UIPageViewC
         super.viewWillAppear(animated)
         self.navigationController?.setToolbarHidden(false, animated: animated)
         courseQuerier.blockWithID(id: blockID).extendLifetimeUntilFirstResult (success:
-            { block in
-                self.environment.analytics.trackScreen(withName: OEXAnalyticsScreenUnitDetail, courseID: self.courseID, value: block.internalName)
+            {[weak self] block in
+                self?.environment.analytics.trackScreen(withName: OEXAnalyticsScreenUnitDetail, courseID: self?.courseID ?? "", value: block.internalName)
             },
             failure: {
                 Logger.logError("ANALYTICS", "Unable to load block: \($0)")

Source/VideoBlockViewController.swift

@@ -104,16 +104,16 @@ class VideoBlockViewController : UIViewController, CourseBlockViewController, St
     }
 
     override func viewDidAppear(_ animated : Bool) {
-
-        loadVideoIfNecessary()
-
+
         // There's a weird OS bug where the bottom layout guide doesn't get set properly until
         // the layout cycle after viewDidAppear so cause a layout cycle
         view.setNeedsUpdateConstraints()
         view.updateConstraintsIfNeeded()
         view.setNeedsLayout()
         view.layoutIfNeeded()
         super.viewDidAppear(animated)
+
+        loadVideoIfNecessary()
 
         if !(environment.interface?.canDownload() ?? false) {
             guard let video = environment.interface?.stateForVideo(withID: blockID, courseID : courseID), video.downloadState == .complete else {