fix: handle Null Dereference in UnitsConverter

.inferconfig

@@ -2,7 +2,6 @@
     "report-block-list-path-regex": [
         "src/main/java/com/thealgorithms/ciphers/a5/CompositeLFSR.java",
         "src/main/java/com/thealgorithms/conversions/RomanToInteger.java",
-        "src/main/java/com/thealgorithms/conversions/UnitsConverter.java",
         "src/main/java/com/thealgorithms/datastructures/crdt/GCounter.java",
         "src/main/java/com/thealgorithms/datastructures/crdt/PNCounter.java",
         "src/main/java/com/thealgorithms/datastructures/graphs/KahnsAlgorithm.java",

src/main/java/com/thealgorithms/conversions/UnitsConverter.java

@@ -3,6 +3,7 @@
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.Set;
 import org.apache.commons.lang3.tuple.Pair;
 
@@ -77,7 +78,7 @@ public double convert(final String inputUnit, final String outputUnit, final dou
             throw new IllegalArgumentException("inputUnit must be different from outputUnit.");
         }
         final var conversionKey = Pair.of(inputUnit, outputUnit);
-        return conversions.get(conversionKey).convert(value);
+        return conversions.computeIfAbsent(conversionKey, k -> { throw new NoSuchElementException("No converter for: " + k); }).convert(value);
     }
 
     public Set<String> availableUnits() {

src/test/java/com/thealgorithms/conversions/UnitsConverterTest.java

@@ -4,6 +4,7 @@
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.Map;
+import java.util.NoSuchElementException;
 import org.apache.commons.lang3.tuple.Pair;
 import org.junit.jupiter.api.Test;
 
@@ -15,4 +16,12 @@ void testConvertThrowsForSameUnits() {
         assertThrows(IllegalArgumentException.class, () -> someConverter.convert("A", "A", 20.0));
         assertThrows(IllegalArgumentException.class, () -> someConverter.convert("B", "B", 20.0));
     }
+
+    @Test
+    void testConvertThrowsForUnknownUnits() {
+        final UnitsConverter someConverter = new UnitsConverter(Map.ofEntries(entry(Pair.of("A", "B"), new AffineConverter(10.0, -20.0))));
+        assertThrows(NoSuchElementException.class, () -> someConverter.convert("A", "X", 20.0));
+        assertThrows(NoSuchElementException.class, () -> someConverter.convert("X", "A", 20.0));
+        assertThrows(NoSuchElementException.class, () -> someConverter.convert("X", "Y", 20.0));
+    }
 }