Assert that mapping is set whenever accessing

// FREEBIE

Signal/src/ViewControllers/SignalsViewController.m

@@ -234,8 +234,6 @@ typedef NS_ENUM(NSInteger, CellState) { kArchiveState, kInboxState };
     [super viewDidLoad];
     [self.navigationController.navigationBar setTranslucent:NO];
 
-    [self tableViewSetUp];
-
     self.editingDbConnection = TSStorageManager.sharedManager.newDatabaseConnection;
 
     // Create the database connection.
@@ -243,6 +241,11 @@ typedef NS_ENUM(NSInteger, CellState) { kArchiveState, kInboxState };
 
     [self showInboxGrouping];
 
+    // because this uses the table data source, `tableViewSetup` must happen
+    // after mappings have been set up in `showInboxGrouping`
+    [self tableViewSetUp];
+
+
     self.segmentedControl = [[UISegmentedControl alloc] initWithItems:@[
         NSLocalizedString(@"WHISPER_NAV_BAR_TITLE", nil),
         NSLocalizedString(@"ARCHIVE_NAV_BAR_TITLE", nil)
@@ -800,6 +803,12 @@ typedef NS_ENUM(NSInteger, CellState) { kArchiveState, kInboxState };
 
 #pragma mark - Groupings
 
+- (YapDatabaseViewMappings *)threadMappings
+{
+    OWSAssert(_threadMappings != nil);
+    return _threadMappings;
+}
+
 - (void)showInboxGrouping
 {
     self.viewingThreadsIn = kInboxState;