Automatic merge of T1.5.1-1695-gade019506 and 17 pull requests

- Pull request #570 at f4a7495: glTF 2.0 support with PBR lighting
- Pull request #900 at 4629736: DMI in 3D cab + two more dials
- Pull request #1030 at d3ae4a2: Refactor settings, in prep for settings exporter
- Pull request #1045 at cc4d53c: Bugfix: Empty Start Time for Explore, and other issues loading from Menu Selection and Content Routes
- Pull request #1052 at 5bbc476: Content Manager: Add axle count, and lowest derail force
- Pull request #1062 at 3b18c48: Train Forces popup Window.
- Pull request #1064 at 9b3415e: Add Train Info tab to Help window (F1)
- Pull request #1066 at 62c89c1: Log derailment, using TraceInformation.
- Pull request #892 at 1f5ba4c: Signal Function OPP_SIG_ID_TRAINPATH
- Pull request #1000 at d8d9709: Locomotive operation from control car
- Pull request #1029 at 92c74ef: Superelevation Follow Up Fixes
- Pull request #1057 at 50c3789: Switchable brake system
- Pull request #1069 at 3f153f0: Mouse wheel control only with Alt – Followup for #1051
- Pull request #1070 at 3a66597: Fix a NullReferenceException in TCS scripts
- Pull request #1055 at f6bf9fa: Route Based TTrack Sounds
- Pull request #896 at f1681df: First implementation of https://blueprints.launchpad.net/or/+spec/specific-sounds-for-ai-trains
- Pull request #1049 at 4dd8c4f: Re-organise document storage and access

Author: openrails-bot

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/TrainControlSystem.cs

@@ -670,9 +670,10 @@ SignalFeatures NextGenericSignalFeatures(string signalFunctionTypeName, int item
                     }
 
                     var functionHead = default(SignalHead);
-                    foreach (var head in trainSignal.SignalObject.SignalHeads)
-                        if (head.Function == function)
-                            functionHead = head;
+                    if (trainSignal?.SignalObject?.SignalHeads != null)
+                        foreach (var head in trainSignal.SignalObject.SignalHeads)
+                            if (head.Function == function)
+                                functionHead = head;
                     signalTypeName = functionHead.SignalTypeName;
                     foreach (var key in functionHead.signalType.DrawStates.Keys)
                     {