Object has been deleted or invalidated

gistfile1.txt

Fatal Exception: RLMException
0  CoreFoundation                 0x1841badb0 __exceptionPreprocess
1  libobjc.A.dylib                0x18381ff80 objc_exception_throw
2  Realm                          0x100b6e300 RLMGetArray(RLMObjectBase*, unsigned long) (RLMAccessor.mm:212)
3  LingQ                          0x1000bdaf0 -[GCCellFactory configureCell:forLesson:locked:feed:] (GCCellFactory.m:173)
4  LingQ                          0x1000f0188 -[LQTableViewController configureCell:section:row:] (LQTableViewController.m:353)
5  LingQ                          0x1000effec -[LQTableViewController configuredCellForTableView:row:indexPath:] (LQTableViewController.m:342)
6  LingQ                          0x1000f086c -[LQTableViewController tableView:cellForRowAtIndexPath:] (LQTableViewController.m:421)
7  UIKit                          0x18966f030 -[UITableView _createPreparedCellForGlobalRow:withIndexPath:willDisplay:]
8  UIKit                          0x18966f198 -[UITableView _createPreparedCellForGlobalRow:willDisplay:]
9  UIKit                          0x18965e298 -[UITableView _updateVisibleCellsNow:isRecursive:]
10 UIKit                          0x189673c64 -[UITableView _performWithCachedTraitCollection:]
11 UIKit                          0x1894048c4 -[UITableView layoutSubviews]
12 UIKit                          0x1893141e4 -[UIView(CALayerDelegate) layoutSublayersOfLayer:]
13 QuartzCore                     0x186ca6994 -[CALayer layoutSublayers]
14 QuartzCore                     0x186ca15d0 CA::Layer::layout_if_needed(CA::Transaction*)
15 QuartzCore                     0x186ca1490 CA::Layer::layout_and_display_if_needed(CA::Transaction*)
16 QuartzCore                     0x186ca0ac0 CA::Context::commit_transaction(CA::Transaction*)
17 QuartzCore                     0x186ca0820 CA::Transaction::commit()
18 QuartzCore                     0x186c99de4 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*)
19 CoreFoundation                 0x184170728 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__
20 CoreFoundation                 0x18416e4cc __CFRunLoopDoObservers
21 CoreFoundation                 0x18416e8fc __CFRunLoopRun
22 CoreFoundation                 0x184098c50 CFRunLoopRunSpecific
23 GraphicsServices               0x185980088 GSEventRunModal
24 UIKit                          0x189382088 UIApplicationMain
25 LingQ                          0x1001374a8 main (main.m:17)
26 libdispatch.dylib              0x183c368b8 (Missing)