[Bug]: MockDesugar throws NPE with legacy function mocking

[Dilhasha]

Description

When using legacy function mocking to mock a function in another module, for a multi-module project, it results in an NPE when trying to find the function to be mocked in MockDesugar level.

Steps to Reproduce

• Checkout the package at https://github.com/Dilhasha/MockIssue
• do bal build
• this throws the below NPE

[2023-06-16 13:47:29,269] SEVERE {b7a.log.crash} - null 
java.lang.NullPointerException
        at org.wso2.ballerinalang.compiler.desugar.MockDesugar.getInvokableSymbol(MockDesugar.java:214)
        at org.wso2.ballerinalang.compiler.desugar.MockDesugar.getImportFunction(MockDesugar.java:196)
        at org.wso2.ballerinalang.compiler.desugar.MockDesugar.generateMockFunction(MockDesugar.java:159)
        at org.wso2.ballerinalang.compiler.desugar.MockDesugar.generateMockFunctions(MockDesugar.java:131)
        at org.wso2.ballerinalang.compiler.desugar.Desugar.visit(Desugar.java:774)

Affected Version(s)

2201.6.0

OS, DB, other environment details and versions

No response

Related area

-> Test Framework

Related issue(s) (optional)

No response

Suggested label(s) (optional)

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Suggested assignee(s) (optional)

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[Dilhasha]

If we add the import dilhashanazeer/app1.dao as a null import, the issue gets resolved.
We need to handle the NPE to provide a meaningful error.

[github-actions]

This issue is NOT closed with a proper Reason/ label. Make sure to add proper reason label before closing. Please add or leave a comment with the proper reason label now.

      - Reason/EngineeringMistake - The issue occurred due to a mistake made in the past.
      - Reason/Regression - The issue has introduced a regression.
      - Reason/MultipleComponentInteraction - Issue occured due to interactions in multiple components.
      - Reason/Complex - Issue occurred due to complex scenario.
      - Reason/Invalid - Issue is invalid.
      - Reason/Other - None of the above cases.