Why do I get an error when using Beamsearh?

[tianzhaotju]

Hello, I have received an error after setting beamsearch, looking forward to your answer.

### The following information is displayed:

Checking out 41ba9e00 to /root/DeepMutation/data/in/Math/106f.............. OK
Init local repository...................................................... OK
Tag post-fix revision...................................................... OK
Excluding broken/flaky tests............................................... OK
Excluding broken/flaky tests............................................... OK
Initialize fixed program version........................................... OK
Apply patch................................................................ OK
Initialize buggy program version........................................... OK
Diff 41ba9e00:5de83dca..................................................... OK
Apply patch................................................................ OK
Tag pre-fix revision....................................................... OK
Check out program version: Math-106f....................................... OK
Extracting methods from data/in/Math/106f/...
Building AST...
done.
Extracted 588 methods.
done.
Writing extracted methods...
done.
Abstracting methods...
There were 0 unparseable methods.
There were 0 methods longer than 1000 tokens.
There are 588 remaining methods.
done.
Writing abstracted methods...
done.
Writing mappings...
done.
Mutating methods...
Using 5 beams.
Running model 50len_ident_lit...
Generated 100 mutated methods.
Generated 200 mutated methods.
Generated 300 mutated methods.
Generated 400 mutated methods.
Generated 500 mutated methods.
Generated 588 mutated methods.
WARNING: could not generate mutated methods. Set MethodMutator.verbose(true) via config file for more details.
done.
Writing abstracted mutants...
ERROR: cannot write null/empty map
Translating abstract mutants...
ERROR: null/empty input map
Writing translated mutants...
ERROR: cannot write null/empty map
Creating mutant files...
ERROR: cannot write null/empty map
MutantTester disabled. Stopping.

### My config:
// Configuration file example.

################## Paths #####################
// Path to output directory
output.path=data/out/Math/106f/

// Path to the project directory
project.path=data/in/Math/106f/

// Path to the src root directory
source.path=data/in/Math/106f/src/java/

// Path to lib directory
library.path=spoonLibs/Math/

// Path to libWrapper.so (generated post-build)
wrapper.library.file=dist/libWrapper.so

// Path to model directories (comma-separated)
model.paths=models/50len_ident_lit/
token.threshold=1000

############## MethodMutator #################
mutator.enable=true
MethodMutator.verbose=true
mutator.verbose=true

// python3 command
python=python3

// verbose
verbose=false

// Use beam search
use.beams=true

// Number of beams
num.beams=5

############# MutantTranslator ###############
translator.enable=true

############## MutantTester ##################
tester.enable=false

// Enable parallel testing
max.threads=8

// Compile command
compile.command=defects4j compile

// Test command
test.command=defects4j test

// Timeout per command (in seconds)
timeout=300

// Remove project copies after testing
clean.up=true

// Baseline is established by running the provided
// commands on a copy of the original project.
// Success/failure is determined by a string diff.
use.baseline=false

// Alternatively, set fail strings that appear in
// command output (comma-separated)
compile.fail.strings=FAIL
test.fail.strings=FAIL,Failing