I have the same problem with IGV 2.9.2 run on Ubuntu 18.04 (32 GB RAM). When I upload gff3 annotation tracks to a genome and save the session, the error appears after re-opening the session. Below is the corresponding info from "igv.log".
INFO [2021-04-12T10:28:57,352] [Main.java:205] Startup IGV Version 2.9.4 03/17/2021 05:06 PM
INFO [2021-04-12T10:28:57,359] [Main.java:206] Java 11.0.10 (build 11.0.10+9) 2021-01-19
INFO [2021-04-12T10:28:57,360] [Main.java:211] JVM: OpenJDK 64-Bit Server VM AdoptOpenJDK
INFO [2021-04-12T10:28:57,387] [Main.java:214] Default User Directory: /home/jirka
INFO [2021-04-12T10:28:57,387] [Main.java:215] OS: Linux 4.15.0-140-generic amd64
INFO [2021-04-12T10:28:57,900] [GenomeManager.java:183] Loading genome: /mnt/raid/454_data/MinION/analysis/centromere_assembly_Cameor/210304_CEN6_all_scaff_and_assembly/all_scaff_210305/igv/CEN6_all_scaff_210305.genome
INFO [2021-04-12T10:28:58,122] [CommandListener.java:121] Listening on port 60151
INFO [2021-04-12T10:29:36,094] [IGV.java:1316] Loading 2 resources.
INFO [2021-04-12T10:29:36,102] [TrackLoader.java:122] Loading resource, path /mnt/raid/454_data/MinION/analysis/centromere_assembly_Cameor/210304_CEN6_all_scaff_and_assembly/all_scaff_210305/repeatmasker_SAT/RM/FabTR-10_cutoff_1kb_track.gff3
INFO [2021-04-12T10:29:36,145] [TrackLoader.java:122] Loading resource, path /mnt/raid/454_data/MinION/analysis/centromere_assembly_Cameor/210304_CEN6_all_scaff_and_assembly/all_scaff_210305/repeatmasker_SAT/RM/FabTR-48_cutoff_1kb_track.gff3
INFO [2021-04-12T10:30:08,498] [ShutdownThread.java:47] Shutting down
INFO [2021-04-12T10:30:32,242] [Main.java:205] Startup IGV Version 2.9.4 03/17/2021 05:06 PM
INFO [2021-04-12T10:30:32,251] [Main.java:206] Java 11.0.10 (build 11.0.10+9) 2021-01-19
INFO [2021-04-12T10:30:32,253] [Main.java:211] JVM: OpenJDK 64-Bit Server VM AdoptOpenJDK
INFO [2021-04-12T10:30:32,287] [Main.java:214] Default User Directory: /home/jirka
INFO [2021-04-12T10:30:32,288] [Main.java:215] OS: Linux 4.15.0-140-generic amd64
INFO [2021-04-12T10:30:32,835] [GenomeManager.java:183] Loading genome: /mnt/raid/454_data/MinION/analysis/centromere_assembly_Cameor/210304_CEN6_all_scaff_and_assembly/all_scaff_210305/igv/CEN6_all_scaff_210305.genome
INFO [2021-04-12T10:30:33,043] [CommandListener.java:121] Listening on port 60151
INFO [2021-04-12T10:30:47,229] [TrackLoader.java:122] Loading resource, path /mnt/ceph/454_data/MinION/analysis/centromere_assembly_Cameor/210304_CEN6_all_scaff_and_assembly/all_scaff_210305/repeatmasker_SAT/RM/FabTR-10_cutoff_1kb_track.gff3
INFO [2021-04-12T10:30:47,229] [TrackLoader.java:122] Loading resource, path /mnt/ceph/454_data/MinION/analysis/centromere_assembly_Cameor/210304_CEN6_all_scaff_and_assembly/all_scaff_210305/repeatmasker_SAT/RM/FabTR-48_cutoff_1kb_track.gff3
INFO [2021-04-12T10:30:47,347] [MessageUtils.java:76] Error loading features for interval: scaff_1722_ver_2:0-10197491 <br>java.lang.NullPointerException
INFO [2021-04-12T10:30:47,347] [MessageUtils.java:76] Error loading features for interval: scaff_1722_ver_2:0-10197491 <br>java.lang.NullPointerException
ERROR [2021-04-12T10:30:52,276] [FeatureTrack.java:739] Error loading features for interval: scaff_1722_ver_2:0-10197491 <br>java.lang.NullPointerException