Kursmappe Crash

[baum-eule]

Wenn ich in "Mein Unterricht" eine bestimmte Kursmappe öffne, stürzt die App ab.
Error:

W/OnBackInvokedCallback(20478): OnBackInvokedCallback is not enabled for the application.
W/OnBackInvokedCallback(20478): Set 'android:enableOnBackInvokedCallback="true"' in the application manifest.
D/EGL_emulation(20478): app_time_stats: avg=60.14ms min=2.63ms max=2251.43ms count=49

======== Exception caught by widgets library =======================================================
The following _TypeError was thrown building:
type 'Null' is not a subtype of type 'String'

When the exception was thrown, this was the stack: 
#0      _CourseOverviewAnsichtState._buildBody.<anonymous closure> (package:sph_plan/view/mein_unterricht/course_overview.dart:124:60)
#1      SliverChildBuilderDelegate.build (package:flutter/src/widgets/scroll_delegate.dart:490:22)
#2      SliverMultiBoxAdaptorElement._build (package:flutter/src/widgets/sliver.dart:829:28)
#3      SliverMultiBoxAdaptorElement.createChild.<anonymous closure> (package:flutter/src/widgets/sliver.dart:843:55)
#4      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2835:19)
#5      SliverMultiBoxAdaptorElement.createChild (package:flutter/src/widgets/sliver.dart:835:12)
#6      RenderSliverMultiBoxAdaptor._createOrObtainChild.<anonymous closure> (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:349:23)
#7      RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:2657:59)
#8      PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:1071:15)
#9      RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:2657:14)
#10     RenderSliverMultiBoxAdaptor._createOrObtainChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:338:5)
#11     RenderSliverMultiBoxAdaptor.addInitialChild (package:flutter/src/rendering/sliver_multi_box_adaptor.dart:424:5)
#12     RenderSliverList.performLayout (package:flutter/src/rendering/sliver_list.dart:77:12)
#13     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#14     RenderSliverEdgeInsetsPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:139:12)
#15     RenderSliverPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:361:11)
#16     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#17     RenderViewportBase.layoutChildSequence (package:flutter/src/rendering/viewport.dart:601:13)
#18     RenderViewport._attemptLayout (package:flutter/src/rendering/viewport.dart:1554:12)
#19     RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1463:20)
#20     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#21     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#22     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#23     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#24     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#25     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#26     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#27     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#28     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#29     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#30     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#31     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#32     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#33     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#34     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#35     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#36     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#37     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#38     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#39     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#40     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#41     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#42     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#43     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#44     _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1434:11)
#45     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#46     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#47     MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:173:12)
#48     _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:1062:7)
#49     MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:237:7)
#50     RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:403:14)
#51     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#52     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#53     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#54     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#55     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#56     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#57     _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1434:11)
#58     RenderObject.layout (package:flutter/src/rendering/object.dart:2546:7)
#59     RenderBox.layout (package:flutter/src/rendering/box.dart:2389:11)
#60     RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:104:21)
#61     RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:2385:7)
#62     PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:1025:18)
#63     PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:1038:15)
#64     RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:591:23)
#65     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:986:13)
#66     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:457:5)
#67     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1325:15)
#68     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1255:9)
#69     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:1113:5)
#70     _invoke (dart:ui/hooks.dart:312:13)
#71     PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:383:5)
#72     _drawFrame (dart:ui/hooks.dart:283:31)
====================================================================================================
D/EGL_emulation(20478): app_time_stats: avg=21.03ms min=3.46ms max=251.92ms count=48
W/FirebaseCrashlytics(20478): Unable to read App Quality Sessions session id.
D/CompatibilityChangeReporter(20478): Compat change id reported: 194532703; UID 10163; state: ENABLED
I/TRuntime.CctTransportBackend(20478): Making request to: https://crashlyticsreports-pa.googleapis.com/v1/firelog/legacy/batchlog
D/TrafficStats(20478): tagSocket(100) with statsTag=0xffffffff, statsUid=-1
I/TRuntime.CctTransportBackend(20478): Status Code: 200

Wenn man die Zeile (package:sph_plan/view/mein_unterricht/course_overview.dart:124:60) auskommentiert, funktioniert es ohne Probleme.